'arethusas' originates from Ancient Greek, specifically the word 'Ἀρέθουσα', where the element is the proper name 'Arethusa' (a mythological nymph).

'Ἀρέθουσα' passed into Latin and later into English as 'Arethusa'; the modern English plural form 'arethusas' is formed regularly from the English proper noun.

Initially the name referred specifically to the mythological nymph Arethusa; over time the name was applied to springs, places, ships, and biological taxa, so the plural now denotes multiple things bearing that name.