When you get inside each other and produce cum
Jeff: Hey wanna fuck?
Samantha: Sure big daddy!
It's a word commonly used among people of all ages, it is a word that is usually used in normal conversation, it can be used as a insult, or can just be used as a normal word.
Guy 1: FUCK! I stubbed my fuckin toe!
Guy 2: fuck man, that shit look like it hurt.