I agree with the other answerer... we need the context. Without it, I'd have a guess it is referring to someone's character of behaviour. If you behave like you've got a broom up your arse/ass (British/American) you are probably rather pompous/inflexible/intolerant/self-important in character. I've met the expression in novels etc. "He behaved as if he had a broom up his arse.... really pompous. I hate people like that!" (And I assume you know that ass is the word for donkey, and the Americans use it for backside/bottom/rectum, whilst the British keep ass for donkey, and use arse for backside) Hope this helps. But please give the context as it could mean 'a cleaning implement inserted into the bottom of your donkey'... BUT I DON'T THINK IT DOES!