An abbreviation of the name Tupac Amaru Shakur, the legendary rap artist shot in a drive-by in Los Vegas in 1996, dying six days later.
'Pac' is used to describe anyone close to the gangster lifestyle and knows what it's like living hard, just like Tupac did.
Mostly used in the negative to describe someone with 'no pac' who has lived an easy suburban life with a nice house.
Yo, you know Shawny went to a private school? No pac, none.