'armhole' is a compound of the words 'arm' and 'hole'; 'arm' comes from Old English 'earm' (from Proto-Germanic '*armuz') meaning 'arm', and 'hole' comes from Old English 'hol' (from Proto-Germanic '*hulaz') meaning 'hollow' or 'opening'.

'arm' developed from Old English 'earm' and related Germanic forms and 'hole' from Old English 'hol'; the compound 'armhole' emerged in later Middle to Modern English as a straightforward compound meaning a hole or opening for the arm.

Initially it literally meant 'a hole for an arm'; over time the compound retained this primary meaning and also gained a colloquial sense referring to the armpit ('the area under the arm').