'backstay' is an English compound formed from 'back' + 'stay'. 'back' comes from Old English 'bæc' meaning 'rear, back', and 'stay' (in the nautical sense) comes from Middle English 'staie' meaning 'a supporting rope or cable'.

'backstay' developed in modern English by combining the words 'back' and 'stay' to name a stay placed toward the back (aft) of a vessel; it appears as a nautical compound in the 18th–19th centuries.

The term originally simply combined the senses of 'back' (rear) and 'stay' (supporting rope); it has retained that specific nautical meaning of 'a stay toward the stern supporting the mast.'