'armpad' originates from Modern English, specifically a compound of 'arm' and 'pad'.

'arm' comes from Old English 'earm' (meaning 'arm'), while 'pad' comes from Middle English 'pad(e)' meaning 'cushion' or 'soft piece'; these elements combined in Modern English to form 'armpad'.

Initially a literal compound meaning 'a pad for the arm'; its modern use continues that basic sense but has extended to include small removable or sewn-in cushions used inside garments for protection or shaping.