'nonisosceles' originates from Modern English, formed by combining the prefix 'non-' (from Latin 'non' meaning 'not') with 'isosceles' (from Greek 'isoskelēs' meaning 'equal-legged').

'isosceles' comes from Greek 'isoskelēs' ('isos' = 'equal' + 'skelḗs/skelos' = 'leg'), passed into Latin/Medieval Latin and then into English as 'isosceles'. The productive English prefix 'non-' was later attached to produce the modern compound 'nonisosceles'.

Originally 'isoskelēs' meant 'equal-legged'; over time 'isosceles' came to denote a triangle with two equal sides, and the compounded form 'nonisosceles' now simply means 'not isosceles' (i.e., lacking two equal sides).