Used to describe something unusually impressive, showy or shiny. Aboriginal women, or "gins", don't usually have very atttractive underwear, if any at all. When they get some new or clean knickers to cover their gin-slits (vaginas), they think they've just won the lottery.
His new car was very showy and shiny. He thought it was the gin's knickers.
My granny just got a promotion at work so she thinks she's the gin's knickers.
The cops just arrested a rock-spider so they thing the've scored the gin's knickers!
That meal was so delicious, it was the gin's knickers!