Gamesmanship is the art of winning
by subtly manipulative
methods that do not in any way constitute cheating
. It is the art of distraction
; of being unnerving
but never annoying; of making your opponent
, perhaps making them
feel bad about being a little unsporting
The word was invented by the English humorist Stephen Potter in his 1947 book The Theory and Practice of Gamesmanship, or, The Art of Winning Games without Actually Cheating. In completely deadpan style he went through the techniques by which one could gain quiet, intangible ascendancy over other players, without having to rely too much on skill.
Naturally this approach is more widely applicable than in games, and Potter followed this up with books on lifemanship (1950), oneupmanship (1952), and supermanship (1958, one I hadn't heard of till beginning to write this). They were popular books, and were further spread by a a 1960 film School for Scoundrels, starring Terry-Thomas as the successful and practised scoundrel, of course, and Ian Carmichael as the new recruit at the College of Lifemanship, an institution Potter invented and claimed to be the source of these self-help manuals.
Good manners are terribly important; they were even more so back in 1947, when it was a serious matter to be in breach of them, or to be in any way lacking in sportsmanship; so one classic ploy is to be excessively good-mannered and sporting. Apologise for doubting their word if they say it was out. If it was clearly out, and you're so good as to retract your doubts so politely, that sets the seed of doubt in their mind for the next time or the one after. Realise you just might be blocking their light, or distracting them, so hop out of the way with a murmured apology: if they weren't distracted before, they will be after a couple of goes of this, and won't be able to complain of your behaviour without looking bad.
Another class of techniques is to break down their calm and confident demeanour with subtly unrelated acts. Forget your tennis racquet, or merely wonder whether you had forgotten it, or merely stop and smite your brow, and then say oh it was nothing, you had momentarily wondered if you had forgotten anything. Your opponent doesn't want to get to the venue then find they have to go back for anything: that little gnawing fear will be working to slow down their reflexes and damp their confidence just a little bit.
It's been a long time since I've seen the books themselves (or the film). My source for memories is: