I don't think it's that common, and I don't think I've ever heard it before. https://books.google.com/ngrams/graph?content=behind+the+eight+ball%2Cbehind+the+8

I don't think it's that common, and I don't think I've ever heard it before. https://books.google.com/ngrams/graph?content=behind+the+eight+ball%2Cbehind+the+8+ball&year_start=1800&year_end=2019&corpus=26&smoothing=3&direct_url=t1%3B%2Cbehind%20the%20eight%20ball%3B%2Cc0%3B.t1%3B%2Cbehind%20the%208%20ball%3B%2Cc0 https://books.google.com/ngrams/graph?content=behind+the+eig... Edit for comparison: https://books.google.com/ngrams/graph?content=behind+the+eight+ball%2Cbehind+the+8+ball%2Cat+a+disadvantage&year_start=1800&year_end=2019&corpus=26&smoothing=3&direct_url=t1%3B%2Cbehind%20the%20eight%20ball%3B%2Cc0%3B.t1%3B%2Cbehind%20the%208%20ball%3B%2Cc0%3B.t1%3B%2Cat%20a%20disadvantage%3B%2Cc0 https://books.google.com/ngrams/graph?content=behind+the+eig...