Losing one's sex drive to someone attractive before a party in order to not do anything sexual with an unattractive person(s) at the party.
When a person knows that they'll do something naughty with someone that they really don't want to, they do a pre-party pinch (sometimes referred to as a pre-pinch).
Rick: I 'eard Chantelle was going to the party at the Chalk, so I'm gunna have a pre party pinch with the babe next door before I go.
Mick: Yeah wouldn't want to have a repeat of last time she was at the same party as you. Don't shit yourself with the neighbour like last time.
Rick: Leave it out.