'One in the front, the rest in the back!'
Where a male sticks four of his fingers up the female's arse and uses his thumb to insert into the vagina/ play with the clit.
Hence all the fingers are in the back and the thumb is up the front. Like a Minibus and it's passengers.
P: "What, that troll? That lives under the bridge?"
D: "Shes not a troll...."
P: "Yeah whatever, carry on."
D: "Ok shut up and let me tell the story, douche."
P: "Look D, I don't want to know more about your anal fetish, and what's a Minibus anyway?"
D: "It's wkd, you put one in the front and the rest in the back!"
P: "That is gross D, did you really have to tell me that? We are in a lecture ffs!"