Medical Definition of fly agaric
: a poisonous mushroom of the genus Amanita (A. muscaria) that has a variably colored but typically bright-red pileus with a warty white scurf on the surface that with the related death cap is responsible for most cases of severe mushroom poisoning, that has been used as a source of poison for flypaper, and that is extensively used chiefly in northeastern Asia as an intoxicant especially for the hallucinatory effects that it produces—called also fly amanita, fly mushroom
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