Basically the 'practise girl'.
The 'rutters' girl that nobody wants but you have sex with to practise for the hotties another night.
Can be of any age so long as you and your friends deem her appearance unsatisfactory
To go rutting:-
When you're out on the lash with the lads and you purposefully aim to find some ruttlidge in order to a) abuse b) have sex with
When driving down a street and spotting some ugly girl, you and your friends proceed to shout: -
preferably in deep agressive voices in order for that girl to know she is unsatisfactory in appearance.