Literally someone who is 'full of crap'
One who boasts or lies - compulsively (to gain attention, popularity or impress a chick or ones mates).
Originated in the late 90s early 2000s from the kiddy show bags you buy at the Sydney Easter Show - the bags are full of lollies and cheap crap... hence... full of crap!
Location: in Osborne Park, Lane Cove on a balcony.
"You know that hot chick that works at foodies servo?"
"I boned her last night!"
"Fuck off show bag!"
A title given to a person who is full of crap
person 1: i can run 100m in 7 secs
person 2: your such a show bag!