He sat on the wet bench and his pants got soaked on both butt-cheeks immediately.

butt-cheeks 是由 butt（臀部）和 cheeks（脸颊）组成的复合词。butt 指"臀部"，cheeks 本意是"脸颊"，在这里用其比喻义指臀部两侧的隆起部分。butt-cheeks 是一种非正式的说法，指"屁股的两边"。