'armchair' originates from English as a compound of 'arm' and 'chair'; 'arm' comes from Old English 'earm' meaning 'arm', and 'chair' ultimately comes from Latin 'cathedra' via Old French 'chaiere'.

'chair' changed from Old French 'chaiere' (from Latin 'cathedra') into Middle English forms and eventually became English 'chair'; the compound 'armchair' arose in English by joining 'arm' + 'chair' to denote a chair with supports for the arms.

Initially, it meant 'a chair with supports for the arms'; over time it kept this primary sense and also developed figurative uses (e.g. 'armchair critic' meaning someone who criticizes from a position of comfort).