Your through, finished, rejected, gone, kaput, end.

When a girl lets u hit it for weed. Or when a girl trades sex for weed.

Yes, it does imply the addressee's doom, but for a slightly different etymology, see the quote below:

Your precious buttocks has now become the center of a very farcical taunt. This phrase implies that "you are going down" in the near future. This phrase being said to you means that your inevitable fate is nearing. It is only half of the full sentence "your ass is grass and i'm gonna mow it." Basically someone is threatening you and you better get the hell outta dodge.

What some people consider the rules of hitchhiking, where the hitchhiker, upon pickup or throughout the trip, must either provide: 1)Gas for the car, 2)Some type of sexual satisfaction, or 3)Some type of drug, usually marijuana.

1.You're dead fool.
2.You better watch it cuz you don't know who you are messing with.

someone is finished, someone is through, someone is done for.