1960s North London (UK) playground slang/shout used to opt out of a game temporarily, or to prevent yourself being tagged in a game. Often it is accompanied be crossing the fingers and the arms in front of your chest and standing still.
The call can also be used to suspend a whole game temporarily, for example when an adult walks across a football pitch
Said to be derived from the Old English "Fain would I..." or "I do not want to..."
"I was about to be tagged, but I shouted "fainites" so he went on and tagged someone else."
"I was just about to boot the ball when an old lady walked across the playground and someone shouted "fainites".