From Old English 'bæc' (rear of the body) — the body part became a direction, spawning one of English's most productive compound families.
Toward the rear; in return or reply; to a previous position or state. Also a noun referring to the rear part of the body.
From Old English 'bæc' (the back, the rear part of the body), from Proto-Germanic *baką, of uncertain further etymology. Some scholars link it to PIE *bʰeg- (to bend), suggesting the back was conceived as the 'bent part' of the body. The adverbial sense ('go back,' 'come back') developed in Middle English from the noun, as spatial direction