The terms “mushroom” and “toadstool” go back centuries and were never precisely defined, nor was there consensus on application. The term “toadstool” was often, but not exclusively, applied to poisonous mushrooms or to those that have the classic umbrella-like cap-and-stem form. Between 1400 and 1600 AD, the terms tadstoles, frogstooles, frogge stoles, tadstooles, tode stoles, toodys hatte, paddockstool, puddockstool, paddocstol, toadstoole, and paddockstooles sometimes were used synonymously with mushrom, mushrum, muscheron, mousheroms, mussheron, or musserouns.
The word has apparent analogies in Dutch padde(n)stoel (toad-stool/chair, mushroom) and German Krötenschwamm (toad-fungus, alt. word for panther cap). In German folklore and old fairy tales, toads are often depicted sitting on toadstool mushrooms and catching, with their tongues, the flies that are said to be drawn to the Fliegenpilz, a German name for the toadstool, meaning “flies’ mushroom”. This is how the mushroom got another of its names, Krötenstuhl (a less-used German name for the mushroom), literally translating to “toad-stool”.
Source: Wikipedia page “Mushroom”, “Etymology” section.