'inhumanize' originates from English, formed from the adjective 'inhuman' plus the verb-forming suffix '-ize'. The adjective 'inhuman' itself is built from the negative prefix 'in-' (not) + 'human' (from Latin 'humanus' meaning 'of man, human'), while '-ize' comes from a productive suffix (Greek -izein via Latin and Old French) used to form verbs meaning 'to make' or 'to render'.

'inhumanize' developed in modern English by analogy with 'humanize' (from French 'humaniser' < Latin 'humanus'). The pattern of adding '-ize' to adjectives to form verbs was common in the 17th–19th centuries, and 'in-' as a negative/privative prefix combined with 'human' produced 'inhuman' before the verbal form 'inhumanize' was regularly used.

Initially, it meant 'to make not human' or 'to deprive of human qualities'; over time the core meaning has remained similar but broadened to include making behavior or conditions cruel, brutal, or otherwise inhumane.