The infinitive helper verb meaning "to do" or "to make". In vulgar or colloquial contexts, it is frequently used to denote the act of having sex.
He laughs, thinking it's a joke. It isn't.
A vulgar slang term referencing the buttocks or anus.
If you are learning Persian or dealing with Persian media, this is a Red Alert word. Using "kos o kon kardan" as a foreigner is highly inadvisable. It does not carry the playful tone that some English speakers get away with when using "shit" or "damn" in a foreign language.