都有屁股的意思，且在詞性上ass和bum都可以當名詞。但是bum還可以當動詞和形容詞。
但ass還有如下意思：
ass：驢子；? 愚蠢的人；還有驢、傻瓜；笨蛋、?屁股、（通過語言或暴力）泄憤，發泄不滿情緒、（粗魯地表示憤怒或異議）去妳的，去死、出洋相;鬧笑話等意思。
bum也有如下意思：
n.流浪者；?屁股；?狂歡作樂；?笨蛋，廢物
例句：Someone's bum is the part of their body which they sit on.
vt.乞討；
例句：A bum is a person who has no permanent home or job and who gets money by working occasionally or by asking people for money.
vi.流浪；?閑蕩；
例句：I think they're just bumming around at the moment, not doing a lot...?
adj.質量低劣的，無價值的；?無價值的；?殘廢的；?很不愉快的