The original recording behind this clip is not documented, so it is best understood as an online meme sound rather than a line from a known film, show or song. The phrase big back comes from internet slang for eating a lot or acting greedy around food, and short video creators adopted it as a quick teasing reaction. It spread through food videos, eating challenges and reaction edits, where a single short line could carry the whole joke. It stays popular because the phrase is easy to recognise, easy to place and instantly understood by viewers who know the slang.