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.'
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?