'unionid' originates from New Latin, specifically from the family name 'Unionidae', which is based on the genus name 'Unio' (Latin 'unio' meaning 'unity' or 'one').

'unionid' changed from the New Latin taxonomic form 'Unionidae' (used to name the family of mussels) and was adopted into English as 'unionid' to denote an individual member of that family.

Initially it referred to forms related to the genus 'Unio'; over time it came to be used in English to mean any freshwater mussel of the family Unionidae.