A simple amalgamation of the words 'Fuck' (to engage in sexual intercourse), and the slang definition of 'Stag'(a raging erection). Originating from the squallid rural town of Gympie in the great south east of Australia, 'Fuckstag' is most commonly used to describe one who is of low moral calibre, social standing or who is poorly regarded by his peers.

Literally everyone, ever. Including the writer of this definition.