'forked-finned' originates from Modern English, formed as a compound of the adjective 'forked' and the adjective/participle 'finned', where 'fork' ultimately derives from Latin 'furca' meaning 'fork' (a pitchfork or two-pronged implement) and the suffix '-ed' marks a past participle/adjectival form; 'fin' derives from Germanic roots (Old English/Old Norse) meaning 'a fish's fin'.

'fork' came into English via Old English/Middle English (from Latin 'furca' through Germanic mediation) and developed into Middle English 'fork(e)'; 'fin' appeared in Old English/Old Norse forms (e.g. Old English 'finn', Old Norse 'finnr') and entered Middle English as 'fin', eventually forming the modern compound adjective 'forked-finned'.

Initially, 'fork' referred to a bifurcating tool or branch and 'fin' to a fish appendage; over time the compound has been used descriptively in zoological and common usage to mean 'having a fin that is split into two lobes', a specialized descriptive term for fish morphology.