'armpit' originates from Old English, specifically the word 'earmpytt', where 'earm' meant 'arm' and 'pytt' meant 'pit' or 'hollow'.

'armpit' changed from the Old English word 'earmpytt' and through Middle English forms like 'arm-pit' eventually became the modern English word 'armpit'.

Initially, it meant 'the hollow under the arm', but over time it also developed an informal figurative use meaning 'a very unpleasant place'.