'retrorse' originates from Latin, specifically the word 'retrorsus', where 're-' meant 'back' and 'torsus' (from 'torquere' / past participle 'tortus') meant 'twisted' or 'turned'.

'retrorse' came into English via Medieval/Scientific Latin 'retrorsus' (used in descriptions) and entered specialized botanical/zoological English usage; the adverbial form 'retrorsely' is formed in English from the adjective 'retrorse'.

Initially it meant 'turned back' in Latin, and over time it has retained that basic sense but become specialized in English usage to describe hairs, spines, or other parts that are bent or directed backward; the adverb simply expresses that manner.