'armplate' is a Modern English compound formed from 'arm' + 'plate'. 'arm' originates from Old English 'earm' meaning 'arm', and 'plate' comes via Old French 'plate' from Late Latin/Medieval Latin (e.g. 'plattus') meaning 'flat' or 'a flat piece'.

'armplate' developed as a compound term in post-medieval English combining 'arm' (Old English 'earm') with 'plate' (Old French 'plate'), used to describe a flat piece (plate) associated with the arm; related medieval armor terminology (e.g. 'vambrace', 'rerebrace') influenced its specific use referring to arm protection.

Initially, the components indicated simply a 'flat piece related to the arm'; over time the compound came to refer specifically to a protective plate for the arm in armor and, by extension in modern usage, to plates attached to mechanical arms.