The word used in the Greek manuscripts of 2 Th 2:3 is apostasia (apostasy), which meaning is "falling away," and which the KJV translated correctly.
Clare, you are partially correct. Apostasia has five definitions, but the two main definitions will serve our purposes.
In the Liddell & Scott Greek-English Lexicon, apostasia's first two definitions are (1) defection/revolt, and (2) departure/disappearance. Please note that "departure" in definition #2 is associated with "disappearance." Now, we know what "disappearance" means, therefore, "departure" can only mean "physical departure." This is not my opinion. These are facts of Greek translation to English.
Now, definition #1 does relate to "falling away." Therefore, we need to consult the text where "apostasia" is found, to see which definition is applicable.
Let's run through 2 Th 2:1-3 in older Bibles:
2 Th 2:1 (Geneva Bible): Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our assembling unto him,
In that verse, Paul is merely raising the topic of the rapture. The reason he had to do that is, his Thessalonian flock had received a fraudulent letter, making it appear to be from Paul, stating that they had missed the rapture and that the Trib had started. Please verse 2 below.
2 Th 2:2 (NLT): Don’t be so easily shaken or alarmed by those who say that the day of the Lord has already begun. Don’t believe them, even if they claim to have had a spiritual vision, a revelation, or a letter supposedly from us.
Those are Paul's words about how his flock had been deceived. Let's look at verse 3.
2 Th 2:3 (Geneva Bible): Let no man deceive you by any means: for that day shall not come, except there come a departing first, and that that man of sin be disclosed, even the son of perdition.
"That day" mentioned in verse 3 concerns "the day of the Lord" mentioned in verse 2. The DOL (Day of the Lord) starts with the Trib. Paul then said "that day shall not come, except there come a departing first, .."
With verse 1 citing the rapture, the context of rapture continues through verse 3. Therefore, of the first two definitions of apostasia, only definition #2 applies: departure/disappearance.
In 2 Th 2:1-3, Paul was factually talking about the pre-Trib rapture. However, this is only clear in early Bibles that have never been corrupted. The KJV came out in 1611 as the first Bible to use definition #1 of apostasia in 2 Th 2:3. That's what started all the Bible versions since then, using definition #1 of apostasia, which factually does not fit the context.
Paul had prior writings on the rapture: 1 Th 1:10, 1 Th 4:16-17 and 1 Th 5:9. Paul had never spoke about a 'falling away" in his prior letters. He repeatedly spoke of the rapture.
Lastly, the "falling away" in the Bible occurs in Matt 24:10. If you research where that verse falls on the timeline, it's well into the Trib, but before the midpoint. Therefore, the "falling away" can't occur in 2 Th 2:3 because verse 3 occurs before the Trib, just as Paul said in verse 3. How can a falling away that occurs near the midpoint of the timeline, be said to occur before the Trib? KJV got it very wrong, and that's very unfortunate. But know this: KJV has also never been willing to say why they made that sea change in verse 3. They are hiding something. Thank God for early Bibles.