He's actually saying Dumbass MARK not mort. White boys born after 1992 shouldn't try to define terms in era video games about the same year.
adj. A person identified as a loser, "buster" or a person who is easy to scam or hustle. Its origin is from that of gangsters and hitmen who would refer to their intended victim as a "mark."
"That fool was a buster-ass mark!"
A word used by Carol Johnson in Grand Theft Auto San Sndreas to describe the driver of a car he is hit by.
It means a stupid person who fucks in the ear and says it was a pussy.
He was a dumbass mort and his friends knew it by what his girl told em