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late 14c., apparently just what it looks like: a fanciful name from Middle English tadde "toad" (see toad) + stole "stool" (see stool). Toads themselves were regarded as highly poisonous, and this word is popularly restricted to inedible or poisonous fungi, as opposed to mushrooms (e.g. toad-cheese, a poisonous fungi).
any of various inedible or poisonous species of mushrooms (kingdom Fungi). See mushroom.