Etymology Dictionary

IM- (1),  prefix.  (F.,—L.; or E.)    A. In some words, im- is a corruption of the O. French prefix em-, but is spelt im- (as sometimes in later F.) by confusion with the Latin prefix im- whence it is derived.   B. And further, by a confusion arising from the double use of the prefix in- (which is both Eng. and Lat.) it was often looked upon as a fair substitute for the E. in, and is prefixed to words of purely E. origin, when the next letter is b or p.   Exx.:  im-bed, im-bitter, im-body, im-bosom, im-bower, im-brown; and similarly im-park.

IM- (2),  prefix.  (L.)   In many words, im- = in-, from the Lat. prep. in, in; the next letter being b, m, or p.   Exx.:  im-bue, im-merge, im-migrate, im-minent, im-mit, im-pel, im-pend, &c.

IM- (3),  prefix.  (F.,—L.)   In some words im- = F. im- = Lat. im-, substituted for in-, negative prefix, when the letter following is b, m, or p.   See In- (3).   Exx.:  im-becile, im-mediate, im-memorial, im-mense, im-modest, im-munity, im-palpable, &c.   And see Im- (1).