There's always one on a boys night out, the one who has to leave early.
"Sorry, but I'm on a promise, lads."
'On a promise' then, means a male has been told he will 'get his oats' if he goes home early.
Well, that's what it's supposed to mean. But we all know the truth is that if he doesn't get home when he should then he's likely to get twatted with the frying pan by her indoors when he does eventually stagger in.
It's the perfect excuse, though, for those married men who just can't handle their drink. After all, who would deprive somebody of their oats.