'cat-poisonous' originates from Modern English, specifically a compound of 'cat' and 'poisonous'; 'cat' ultimately comes from Old English 'catt' (meaning 'cat'), and 'poisonous' comes from Old French 'poison' (from Latin 'potio'), where 'potio' meant 'a drink' or 'poison'.

'cat-poisonous' developed from the earlier descriptive phrase 'cat poisonous' (two words) in English and later appeared hyphenated as 'cat-poisonous' in modern usage.

Initially it simply combined the senses of 'cat' + 'poisonous' ('poisonous to cats'); over time the hyphenated compound has kept this same specific meaning of 'toxic to cats'.