a rude phrase meaning to force yourself to start doing something or to make yourself hurry

Tell him to get off his arse and do some work for once. 告诉他别再干坐那儿不动了，他总得有那么一次干点活吧。

If she doesn't get her arse in gear, she's going to be late. 如果她再继续磨蹭，她就要迟到了。