n. & v.
n. 1 the usu. edible spore-producing body of various fungi, esp. Agaricus campestris, with a stem and domed cap, proverbial for its rapid growth. 2 the pinkish-brown colour of this. 3 any item resembling a mushroom in shape (darning mushroom). 4 (usu. attrib.) something that appears or develops suddenly or is ephemeral; an upstart.
v.intr. 1 appear or develop rapidly. 2 expand and flatten like a mushroom cap. 3 gather mushrooms. mushroom cloud a cloud suggesting the shape of a mushroom, esp. from a nuclear explosion. mushroom growth 1 a sudden development or expansion. 2 anything undergoing this. mushroomy adj.
[ ME f. OF mousseron f. LL mussirio -onis ]