to kill, steal, rape, pillage, etc.
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.