Named after a popular South African sport-fish (Spotted Grunter - Pomadasys commersonni) which is known for grunting like a pig when landed, a grunter is a very unattractive woman.
The term is so widely used that young men
developed a game called 'Hunt the Grunt' whereby each player adds money to a pot and the aim is to score
the ugliest girl/woman possible on any
one night. At the end a winner is decided by popular (unpopular) vote and he takes the pot. It is then his decision whether or not to buy his grunter a drink with his winnings.