used to describe either;
a) a male/female with an horrificly ugly face or
b) someone with an angry face similar to a gurn
derived from two seperate cockney rhyming phrases;
1)Grumble=Grumble and Grunt=Cunt
so the translation is clearly cuntface...
'look at the GrumbleBoat on that...she was grim!'
'That dude was livid, he had a proper GrumbleBoat on him'