Often used to express emotion about an imaginary person. Muhammad dammit means to say that something was funny, surprising, or crazy.
Person 1: *Drops a bucket of paint on third floor.*
Person 2: Muhammad dammit! Watch out, you almost dropped that on me!
Person 1: Hey, don't say that! That hurts my feelings! I'm gonna hit you because my "religion of peace" says to do it to non believers.
Person 2: I'd like to see you try.
Person 1: *Punches like a pussy ass little girl.*
Person 2: *Pimp slaps the cunt pedo worshiper.*
Person 3: Islamophobia!
Person 2: He tried to hit me first.
Person 3: It's his right you islamophobe! #freepaleshit
Person 2: SMH I'm sorry your parents dropped you when you were a baby.
Person 3: Duh I want to be like Muhammad. Where the kids at?