This Greek word is found in 2 Thessalonians 2:6–7 in an eschatological context: Christians must not be concerned as if the Day of the Lord would happen tomorrow, since the man of lawlessness (Antichrist) must be revealed before it occurs.
St. Paul the Apostle then adds that the Antichrist's emergence is conditional upon the removal of "something/someone that restrains him" from becoming fully manifested. Verse 6 uses the neuter gender, τὸ κατέχον; and verse 7 the masculine, ὁ κατέχων.
Since Paul does not explicitly mention the katechon's identity, the passage's interpretation has been subject to extensive dialogue and debate amongst Christian academics.
BDAG has eight different meanings assigned to κατέχω, but all surround the idea if keeping in one's possession of preventing something by restraining.
In the case of 1 Thess 2:6, 7, BDAG has a lengthy paragraph of explanation beginning with this:
that which restrains, and the one who restrains, ie, what prevents God's adversary from coming out in open opposition to God, for the time being. ...
The biggest problem with the proposed translation of the OP is that such a meaning cannot be found anywhere else either in the NT or in 1st cent Koine Greek literature, according to BDAG. Thus, "prevail", while quoted by the dated LSJ (quoting classical sources for the innovative meaning), does not accord with Koine Greek usage in the 1st century. Thus, there is a very good reason why no modern translations adopt the meaning "prevail".
The key to understanding this passage in 2 Thess 2 is V3, & V4
3 No one is to deceive you in any way! For it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction, 4 who opposes and exalts himself above [e]every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God.
Paul then expands the point by saying (NASB):
6 And you know what restrains him now, so that he will be revealed in his time. 7 For the mystery of lawlessness is already at work; only He who now restrains will do so until He is removed. 8 Then that lawless one will be revealed, whom the Lord will eliminate with the breath of His mouth and bring to an end by the appearance of His coming; 9 that is, the one whose coming is in accord with the activity of Satan, with all power and false signs and wonders, 10 and with all the deception of wickedness for those who perish, because they did not accept the love of the truth so as to be saved. 11 For this reason God will send upon them a deluding influence so that they will believe what is false, 12 in order that they all may be judged who did not believe the truth, but took pleasure in wickedness.
Paul is predicting a time of general apostasy in the church, caused by the "man of lawlessness" who is currently being restrained. He would be revealed to exposed him fully and that revelation of lawlessness will be complete when Jesus returns.
Translating κατεχω as “to restrain” at 2 Thess. 2:6-7
Allow me to try and answer this question using my translation (NASB):
2 Thessalonians 2:6-7: "And you know what is now restraining him, so that he may be revealed at the proper time. For the mystery of lawlessness is already at work, but the one who now restrains it will continue until he is taken out of the way.
Answer:
The N/T did not yet exist and, thus, (NT) lawlessness in all its forms was "restrained". When progressively recorded, so did the emergence of lawlessness against it, and those who rejected the Message would be revealed as transgressors.
What is 'the mystery of lawlessness' -- 'the man of lawlessness,' what 'is restraining' him now and who/what does the restraining?"
To understand what is being related by Paul, I believe it is crucial to understand the verses that precede these two for context:
2 Thessalonians 2:1, 3-4 "Now concerning the coming of our Lord Jesus Christ... 3Let no one deceive you in any way, for it will not come until the rebellion occurs and the man of lawlessness—the son of destruction—is revealed. 4He will oppose and exalt himself above every so-called god or object of worship. So he will seat himself in the temple of God, proclaiming himself to be God. 5Do you not remember that I told you these things while I was still with you?"
Many will (understandably) tie this demonic force, as suggested in the OP, to "the power of the Roman empire" and "the one in whom that power is lodged, the Roman emperor." Others propose that this wicked figure is the Roman ecclesiastical movement. Of the two, the initial thrust of Paul's Letter may be directed at Rome, and the Caesars. However, should we limit such words only to these two identities?
First, we must recognize that during this time in history -- the first century, the Gospel did not yet exist. And, although many false teachers were about to emerge (2 Peter 2:1) and later did so (Jude vs. 4, 1 Jn. 2:18: "many antichrists", 2 Jn. 1:7: "deceiver and antichrist", etc.), they were "restrained" by the simple fact that there was insufficient Scripture to undermine and blaspheme.
Its absence constituted a natural "restraining force." The "man of lawlessness" in verse 3 may simply represent the emergent figure of the satanic rejection of Christ, and of those who were increasingly able to pervert the Message of God. Of course, it is hard not to see Rome and Caesar in view here as well.
It may be this satanic force (possibly Rome) that is being personified as "the man of lawlessness." Let us reflect on the next passages of 2 Thessalonians 2 in this regard:
2 Thessalonians 2:8-12: "And then the lawless one will be revealed, whom the Lord Jesus will slay [condemn by] the breath of His mouth [Scripture] and annihilate by the majesty of His arrival [judgment]. 9The coming of the lawless one will be accompanied by the working of Satan, with every kind of power, sign, and false wonder,
10and with every wicked deception directed against those who are perishing, because they refused the love of the truth that would have saved them. 11For this reason God will send them a powerful delusion so that they believe the lie, 12in order that judgment may come upon all who have disbelieved the truth and delighted in wickedness."
Elsewhere, I discussed the fact that Christ speaks to such rebellious human beings as follows:
Matthew 7:23: "[I, Christ] will declare to [the disobedient], ‘I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.’"
Interesting. In the first centuury, the "lawless one" could not be revealed as long as the Holy Text had yet to be recorded. As each Book and each Letter of the Bible became a documented reality, Satan was then able to manipulate those who were predisposed to all manner of "powers, signs, and false wonders, and with every wicked deception..." due to their sustained rejection of the Gospel.
Further, we might think of "the man" in terms of the symbolism expressed in Ezekiel's valley of dry bones:
Ezekiel 37:3-6: "[The Lord] said to [Ezekiel]... He said to me, 'Prophesy over these bones and say to them, "O dry bones, hear the word of the LORD."
Thus says the Lord GOD to these bones, "Behold, I will cause breath to enter you that you may come to life. I will put sinews on you, make flesh grow back on you, cover you with skin and put breath in you that you may come alive..."'”
Paul's admonition may simply mean that the "man of lawlessness" would grow in significance as the Gospel was delivered to humanity, many of whom would reject the message. Again, however, in the context of the first century it is indeed difficult to rule out Rome and its blasphemous, satanic rulers.
In totality, the lost may then represent "The Messenger/Man of sin/Satan." We see this today as all manner of false teaching exists. And, the day will come when judgment will occur to those who may, collectively, represent the "mystery of lawlessness" and "the man of lawlessness" although the latter could describe which ever Roman leader was in power at that time.
The absence of N/T Scripture did the restraining, and the delivery of the Message would reveal those who rejected it -- including the godlessness of Rome and its rulers.