A mad yock is a girl who tends to like a bit of cock in her. i.e. a slut or a whore
Jeeze that (girls name) is a mad yock, she has fucked 4 of my friends last week and she i stirred the porridge last night.
Sally: oh i was out on the pish last night and i seen terry who has the 10 inch cock on the dance floor
Kelly: i am frightened by guys with big cocks, last time i had an 8 incher i couldn't walk for a week,
Sally: well i couldn't resist. i went over like the biggest whore and put the moves on him and made him take me home and give me a good fucking in my ass and in my cunt.
Kelly: Fucking hell Sally you are a Mad Yock