A wide-eyed neophyte asks a seasoned spy/soldier/bureaucrat a question about something secret, and this is the hackneyed response. But who said it first, in what movie, TV show, book, etc.?
Tom Cruise said it Top Gun so someone has to beat 1986 now.
Well, I could tell you…
There are posts online linking to Dr. No and the MASH TV series, but no cites.
IMDB brings up a couple more movie results in a search of their quotes database but both were released after Top Gun.
It’s a perfect line for the Col. Flagg character from MASH, it wouldn’t surprise me if it were written for him.
I don’t have an exact date, but I clearly remember hearing Jan-Michael Vincent say that line in the TV pilot of “Airwolf.” I think it was in 1984, but I am positive it was before “Top Gun.”
So, moved only a couple of years back, but that’s all I got.
It’s a standard **magician’s ** line. Probably rooted in the unwritten rule of magicians not telling anyone outside of the craft their secrets.
Spectator: Wow! How did you do that?
Magician: I could tell you. But then I’d have to kill you.
Spectator: Okay, then just tell my wife.
I also believe it was Colonel Flagg on MAS*H.
I think it was the same movie that had the line, “That’s the beauty of it–It doesn’t DO anything.”
I could kill you, but I’d have to tell you.
Why are the Google ads in this thread about drug detox and “dope” ringtones? 
I can’t find that quote from him anywhere. The closest I can find is, “I can kill you with one finger.”
I do know that it was pre-Top Gun. I got mad that he was getting such mileage out of something that had been used quite frequently previous to that.
Don’t know where, tho.
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