'backslap' originates from English, a compound of the words 'back' + 'slap', where 'back' referred to the rear part of the body and 'slap' was an onomatopoeic/imitative word for a striking blow.

'back' comes from Old English 'bæc' and 'slap' appeared in Middle English (with possible Scandinavian influence or onomatopoeic origin); the compound 'backslap' developed in modern English to denote a slap on the back and later the figurative sense of congratulation or mutual praise.

Initially it referred literally to a slap on someone's back; over time it also acquired the figurative meaning of giving praise or engaging in mutual congratulation, sometimes implying insincerity.