Literal definition is, intoxicated female sexual organ. When the term is used colloquially it refers to an intoxicated individual. The term can be abbreviated to, 'WG'.
He hit it well hard over the weekend, what an epic waste gash.
A wasteful vagina. Often belongs to someone of a hideous frame and face, therefore a perfectly good vagina could be wasted because no one would want to fuck a beast.
Boy: yeah bruv that girl iz waste gash.
Boy2; yeah i know so is her mutha
boy1: so what your mum is waste gash to.
Boy 2: thats well harsh, yo mum is a sket cadet, fool.