'toonish' originates from Modern English, specifically the clipped form 'toon' (from 'cartoon') combined with the suffix '-ish' (from Old English '-isc'), where 'toon' referred informally to 'cartoon' and '-ish' meant 'having the quality of'.

'toon' is a clipping of 'cartoon', which entered English from Italian 'cartone' (meaning 'heavy paper' or 'cardboard' used for drawings); 'cartone' → 'cartoon' in English, and later 'toon' as an informal short form. The suffix '-ish' derives from Old English '-isc' via Middle English '-isch' and developed the meaning 'pertaining to' or 'having the quality of'.

Initially, 'cartoon' referred to a drawing on 'cartone' (heavy paper); over time 'toon' became an informal form of 'cartoon', and 'toonish' developed to describe qualities like 'cartoony'—simplified, exaggerated features—now generally used to mean 'cartoony' or 'cartoonishly exaggerated'.