Yes, lots of meanings here! "To hit someone" means simply to strike them. "To hit on someone" means to flirt (thanks Leo). "To do a hit on someone" means to kill them (ie. Jura's hitman). "To hit a bank/store" means to rob it. To say "I'd hit that" about someone means you'd like to have sex with them (Thanks Amy - this was the meaning I thought Ayu was asking about). Yes I think this meaning is a bit crass also. "To hit someone up (for something)" is a slangy way of asking someone for something. Eg. "I can hit my brother up for some money" (=I can ask my brother to give me some money). To ask for the money before doing a job, you could say "hit me" and put your hand out. This is also slang, be careful in case you're understood literally! O_@ I think Ramesh is thinking of "to get knocked up" as a slang expression for getting pregnant. Not really related to hitting. "I got knocked up by my last boyfriend." Or, "I accidentally knocked her up" (=I accidentally got her pregnant.) Again, don't use this phrase in polite situations, it simply makes the speaker sound a bit dumb.

It comes from fighting actually. To hit someone or to kick someone. When you add the preposition "on" with it, the meanings become " to make a sexual comment to someone". For example: That guy tried to hit on my girlfriend, so I punched him in the face. But taking it in the context of vulgarity, it is a slang always. Your question"hit someone" simply means " kick someone".

A hitman is person hired to murder someone.

It is vulgar in the sense " making someonr pregnant " and the other meaning is injurind? hurting...

I agree that you need to be context. It can be vulgar if a guy said "I hit that" about a girl, but, then again, it all depends on the context.