'annelidous' originates from English, specifically formed from the word 'annelid' (from New Latin 'Annelida') plus the adjectival suffix '-ous', where the suffix '-ous' meant 'having the quality of'.

'annelidous' changed from the earlier English noun/adjective 'annelid', which derives from New Latin 'Annelida', from Latin 'annellus' meaning 'little ring' (ultimately from Latin 'anulus' 'ring'), and was extended in English with '-ous' to form an adjective.

Initially it referred to being 'ringed' or 'having rings'; over time it evolved into its current sense of 'relating to annelids (segmented worms)'.