im-youdaoicibaDictYouDict[im- 词源字典]
variant of in- before -b-, -m-, -p-, in the sense of "not, opposite of" (immobile, impersonal) as well as "in, into" (implant, impoverish). See in-. In some English words it alternates with em- (1).[im- etymology, im- origin, 英语词源]