2. A state of first being stabbed in the chest, followed by being anal raped.
3. Showing a sense of ownership over an item, followed by dissing it.
Person A: Dude! Headshot!
Person B: Merked!
Person A: Dude, I just got merked!
Person B: Damn, that sucks.
Person A: Dude, your dog just got merked!
Although other people are offering plausible etymologies for this word, I'm guessing it more likely is a "verbing" of a shortened version of mercenary, or soldier for hire. Just like narcotics officers became narcs, which then became a slang verb for snitching to cops, I think mercenary became merks, then merked became a slang verb for acting like a mercenary, or killing, robbing, etc.