Verb: To take part in the consumption of a substance from a single, usually communal, source. Mainly used to refer to the smoking of marijuana (to "hit" the bong), but can be used in many other circumstances when substances are moving between people (ie: "to hit up").
Noun: A single dose, or small act of consumption, of a larger substance.
1. Yo nigga, you need to hit this joint NOW!
2. Oh, no way! You have to let me hit up that pint of ice cream with you!
3. Damn, I gots some crazy drymouth right about now. Hey, lemme get a hit of that beer.