'open-necked' originates from Modern English as a compound of 'open' + 'necked', where 'open' means 'not closed' and 'necked' relates to the 'neck' or neckline.

'open' comes from Old English 'open' (meaning 'not closed, exposed') and 'neck' comes from Old English 'hnecca' (via Middle English 'nekke'); the adjective-forming suffix '-ed' has been used since Middle English to form adjectives meaning 'having or characterized by'. These elements combined in Modern English to form the descriptive compound 'open-necked'.

Initially it simply described a garment 'with an open neck'; over time it retained that basic meaning and is still used to describe garments or styles where the neckline is open or unbuttoned.