When you fall asleep immediately after ejaculating inside a female while still inside her, and she leaves you asleep on the couch, naked, for her roommates to see when they come back home.
The next morning after waking up naked on her couch, you call your buddy to tell him what happened and you start the story with "You've Probably Done This..."
When a group of co-workers go to the local bar for 1 drink after a long work day. One and done's usually involve trash talking and extremely inappropriate conversations that make other people at the bar feel uncomfortable. It is very common that the participants of one and done will have more than just one drink. However, the term was created to make people feel better about their life choices