-You fool, don't move.. - the guy said while you were sitting on his hips intermittently fidgeting, which made him horny.

his arms wrapped around your waist, holding you in place in order to make it difficult for you to move so that no one noticed his "riser"

it all started with the fact that during a school trip the bus was packed and you had to sit on your boyfriend's lap. but trying to get comfortable on him, you "inadvertently" touched him, which is why he was angry at you now