Mean Sonofabitch (Mean Son of a Bitch) is a first generation super mutant living in Westside, usually found outside the Westside Co-op. Sonofabitch is one of Master's original super mutants who was captured at the Hub by the New California Republic, and while in captivity was tortured by the NCR troopers and had part of his tongue cut out. Later, Tabitha's super mutants freed him from the Hub; his freedom enabled him to venture to Westside, supposedly looking for Klamath Bob, who had tried to buy him from the Hub to stop the NCR troopers from torturing him. He spends his days protecting Westside from hostiles; his very presence is enough to frighten off the Fiends.
Like many of the residents of Westside, Mean Sonofabitch will sometimes say, he is glad Mean Sonofabitch is on "our side" and other normal dialogue when approached by the Courier.
When Lily is in company, some non-player characters will question if she is Mean Sonofabitch, remarking that all super mutants look the same.
When the Courier asks him what he is doing "in a place like this", he remarks something along the lines of protecting Westside from Fiends. It appears the voice actor didn't realize the slurred "Wesibe" in the script was supposed to sound like "Westside," so instead it sounds like "weh-see-beh".
No matter what dialogue option the player chooses, the super mutant will always end with saying goodbye (or rather "Goobye").
Though his name implies him to be very aggressive, he seems to be rather calm and polite, and is usually tolerant around humans.
When talking to Motor-Runner about why the Fiends went into the vault, he will say it was hard to keep his people out of Westside and then say that Mean Sonofabitch would keep killing them.
When asked about his own name, he will call himself "Mahsofahbich." The Courier cannot do a thing about this; it only serves to further exaggerate his speech impediment.
He, along with Dean Domino and God, is one of the only humanoid characters who slowly regenerates health.
He and Marcus are the only two super mutants appearing in any game who have "Good" Karma.
He seems to have retained most of his intelligence from when he was human and can carry out a civil conversation with the Courier, although his speech impediment makes understanding him difficult.