'coat-of-mail' is a compound of 'coat' and 'mail'. 'coat' originates from Old North French, specifically the word 'cote' (from Medieval Latin 'cotta'), where 'cotta' meant 'tunic' or 'outer garment'. 'mail' originates from Old French 'maille', meaning 'mesh' or 'net'.

'coat-of-mail' appears in Middle English in forms such as 'cote-maille' or 'cote of mail', and over time the compound evolved into the modern form 'coat-of-mail'.

Initially it referred specifically to a garment (a tunic) made of metal mesh for bodily protection; this core meaning has largely remained, though it is sometimes used figuratively now to mean any protective layer.