A person or thing that causes backwash; something that creates a backward flow of water or unwanted consequences.
From 'backwash' + the agent suffix '-er' (from Old English -ere), creating a noun meaning 'one who or that which backwashes,' likely coined in the 20th century.
The '-er' suffix is one of English's most productive word-building tools—we can add it to almost any verb to create 'the thing that does that action,' which is why a boat becomes a backwasher when it creates that backward spray.
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