'rudimentary-finned' originates from modern English as a compound of 'rudimentary' and 'finned', where 'rudimentary' ultimately comes from Latin 'rudimentum' meaning 'beginning, first attempt' and 'finned' derives from Old English 'finn' meaning 'fin (of a fish)'.

'rudimentary' passed from Latin 'rudimentum' through Middle English 'rudiment' into modern English 'rudimentary'; 'fin' evolved from Old English 'finn' (and related Germanic forms) into modern English 'fin'; the compound form 'rudimentary-finned' is a recent descriptive formation in modern English combining these elements.

Initially the components referred separately to 'beginning/undeveloped' and 'fin'; when combined they have consistently conveyed the meaning 'having undeveloped fins', a sense that has remained stable in technical and descriptive use.