The late Tupac Shakur defined the fraze M.O.B to mean 'money of bitches', although this definition had a meaning of its own, what tupac actually ment when he said it was that he was now MOB related, i.e, a Member Of Bloods. The Bloods are a west LA street gang.
Therefore the fraze is actually a clever double meaning.
Some rappers still use the fraze 'MOB' to mean Tupacs defined meaning 'Money Over Bitches' without knowing what he was truly implying.
"M.O.B., nigga 'cause we mob on you tricks" - Tupac Shakur