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flat as a shitcarter's hat
A description of the conditions of water when it is very flat, as an analogy to the people who removed pans from toilets and carried them on their heads with possibly a hat or towel to cushion the weight of the pan and hence flattening it: To describe the conditions of water and the definite possibility of going boating.
Contributor's comments: That's how I heard it growing up in south eastern Melbourne in the 1960s.