To clean the oil from (wool) after combing.
To clean (a filter, a chromatography column, a water softener, etc.) by passing liquid through in the reverse of the normal direction of flow. This procedure dislodges particles which clog the pores of the filter or column packing, removing them from the filter or column.
The flow of water propelled backward by the propeller, paddle wheel, or oars of a boat.
The backward flowing air within the wash of an airplane, caused mostly by the engine.