'in-' originates from Latin, specifically the word 'in-', where 'in-' carried meanings including 'in, into' and (in many contexts) 'not' (negative).

'in-' passed into Old French and Middle English as a productive prefix; both the locative sense ('in, into') and the negative sense ('not') were retained, and assimilated forms (im-, il-, ir-) developed before certain consonants.

Initially it primarily conveyed the locative sense 'in, into' in early Latin and Proto-Indo-European derivatives, but in Classical Latin the prefix was also used for negation; English derivatives preserve both senses.