cereal backwash Share definition Flag The part of dry cereal that ends up in the box rather than the inner bag due to slipshod pouring of said cereal. This Fruit Loops box has some serious cereal backwash issues! cereal backwash by ForestDad December 30, 2021 0 0 Get merch Get the cereal backwash mug. Buy Now — $32.95 See more merch