Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
im- (prefix)
is an assimilated adaptation of the prefix in-, one of whose meanings negates the rest of the word (accurate becomes inaccurate). Assimilation causes in- to become im- before a following bilabial consonant such as m (immaterial) or p (improper). When the prefixes in- and en- have other meanings, assimilation also takes place before a following bilabial, such as b (embroil, emboss), m (immure), and p (impel, implore). See also IL-; IN-; IR-.
