'onion-shaped' originates from Modern English as a compound of 'onion' + 'shaped' (the past-participle adjective 'shaped' meaning 'having the form of'). 'onion' itself comes into English via Old French 'oignon' from Late Latin 'unio'/'unionem', and 'shape' derives from Old English 'sceāp'/'scēapian' (from Proto-Germanic *skapaną) meaning 'to form'.

'onion' came into Middle English as forms like 'unyoun'/'onion' from Old French 'oignon', which descended from Latin 'unio' (meaning 'single' or a kind of bulb); 'shape' comes from Old English 'sceapian' (to form) and developed into the noun/adjective forms in Middle and Modern English; the compound 'onion-shaped' is a relatively recent Modern English descriptive formation combining the noun and a past-participle adjective.

Initially the components referred simply to the vegetable 'onion' and the concept 'shaped'; over time the compound came to be used specifically to describe bulbous forms (notably architectural domes) that resemble an onion's rounded profile.