擦屁股 is a separable verb-object compound with both a literal sense (wiping after defecating) and a common figurative sense (cleaning up a mess someone else left, especially in work or social situations). It's an informal, often negative expression — you use it when you have to fix someone else's careless mistake. As a 离合词, elements like 了, 过, or duration phrases slot between 擦 and 屁股 (e.g., 擦了屁股, 擦过屁股). The figurative meaning is far more frequent in adult conversation than the literal one.