If there was any intentionality in the letters TK or FN, I don't know of it. In the case of FN, my recollection is his name Finn came long before the idea that Poe named him was developed. So "FN" was reverse-engineered from Finn. As for TK...
https://twitter.com/Tchalla_Fett/status/1424174487201673221
I am doubtful it was intended to mean anything specific, because the script had it as TX. It was changed to TK during production. Don't know why. But if you look at early sources, it was TK-421. The novelization ('76) even went so far as to use THX-1138 as the trooper number. 
In the years since, in large part due to the work of the @501stLegion, the term "TK" has become a standardized way of discussing stormtrooper numbers. The Mandalorian, I think, was the first place to use "TK number" in dialog.
But what TK or FN means, I don't think has been retconned to have specific meaning.
* TX-421. (Did my autocorrect actually have TK-421 in it??? hah. Or i'm on autopilot.)
(Opinions vary, but if asked, I’d be of the don’t define it camp. You’d risk putting limitations on something that doesn’t appear to have many.)
• • •
Missing some Tweet in this thread? You can try to
force a refresh
