A person who stays out all night shagging or generally up to no good! An affectionate, jokey expression originated in the North of England to describe slutty girls and boys.
Where have you been all night? You dirty stop out!
A person who has not slept in their own bed on a specific night, with the inference being that they had sex, probably with someone met that evening (in a bar, club, wherever).
Where the hell were you last night, you dirty stop-out? Did you go home with that guy/girl/goat/etc?
Probably Australian. Meaning someone who stays out later than expected having a good time.
'Hey you're home. I was just on the phone to ____ telling her you're a dirty stop-out.'