'broad-finned' originates from Modern English, specifically the words 'broad' and 'finned', where 'broad' comes from Old English 'brād' meaning 'wide' and 'finned' is the adjectival form of 'fin' (from Late Middle English 'fin(e)') meaning 'a limb or projection used for swimming.'

'broad' changed from Old English word 'brād' and 'fin' from Middle English 'fin(e)'; the compound 'broad-finned' developed in Modern English by combining these elements to describe animals with wide fins.

Initially it meant 'having wide fins,' and over time this descriptive meaning has remained largely unchanged into its current usage.