Lewis & Short

2. in (before b and p, im; before l, m, and r, the n assimilates itself to these consonants), an inseparable particle [kindred with Sanscr. a-, an-; Gr. ἀ-, ἀν; Goth. and Germ. un-], which negatives the meaning of the noun or participle with which it is connected; Engl. un-, in-, not: impar, unequal: intolerabilis, unbearable, intolerable: immitis, not mild, rude, etc.