Similar to a blumpkin, whereby a male taking a crap receives a blowjob, only this time it's from his sister.
"George, how larng are you going to be in that bathroom? The cow needs milking."
"Sorry Mavis, oi'm just saying g'boi to me sister afore she AAARRRHGGHHH oi told you nart to use yer teeth ya mangy old panhandler! Yer as bad as mother!"