A shortening of the phrase "fuck off".

A shortened term meaning face off. Many people confuse this for a harsh, insulting word when really it means someone wants to engage in a battle.

To not want to have sex

To not want to have sex
F off : no sex
F on : sex

the short for fuck off! since in most cultures 'fuck' is still a word looked down upon , but most people feel like asking someone to fuck off!

hence, the word F-off, can be called Foff too!

When somebody tells you to f-off they are telling you to leave them alone. They don't want you there right now