Why was "tantrum" trending last week?
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- 1pref. Variant of in-1.
im- 2pref. Variant of in-2.