'cytotoxin' originates from Greek, specifically the combining form 'cyto-' from Greek 'kytos' meaning 'cell' and 'toxin' from Greek 'toxikon' meaning 'poison'.

'cytotoxin' formed as a compound of the Modern Latin/Greek combining form 'cyto-' (from Greek 'kytos') and the word 'toxin' (from Greek 'toxikon' via Latin 'toxicum'), and entered modern English usage as 'cytotoxin'.

Initially, it meant 'a poison relating to cells', and over time it has retained this specialized meaning as 'a substance toxic to cells'.