'backbend' is a compound formed from the English words 'back' and 'bend'; 'back' refers to the rear part of the body and 'bend' means to curve or flex.

'back' comes from Old English 'bæc' meaning the rear part, and 'bend' comes from Old English 'bendian'/'bendan' meaning to cause to bend; the modern compound 'backbend' developed by straightforward compounding in Modern English.

Originally each element meant 'rear' and 'to curve' respectively; combined they came to denote specifically the action or posture of curving the body backward and later the named gymnastic/yoga posture.