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.
A:Bro, that thing you scored last night was an absolute grunter!
B: Get fucked, meat is meat and a man must eat.