'rounded-finned' is a Modern English compound formed from 'rounded' (the adjectival/past-participial form of 'round') and 'finned' (the adjectival/past-participial form of 'fin'). 'round' ultimately comes from Old French 'rond', from Latin 'rotundus' meaning 'round'; 'fin' comes from Old English 'finn' (related to Proto-Germanic *finnô) meaning a fin or fin-like appendage.

'rounded' developed by adding the -ed suffix to 'round' (Old French 'rond' → Middle English 'round'), and 'finned' developed by adding -ed to 'fin' (Old English 'finn'); the compound 'rounded-finned' arose in Modern English as a descriptive compound combining those two adjectival forms.

Originally the elements referred simply to 'round' (shape) and 'fin' (a fish appendage); over time the compound came to be used specifically as a concise descriptor meaning 'having rounded fins'.