toadstool (n.)youdaoicibaDictYouDict
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 poisonous or inedible fungi, as distinct from edible "mushrooms" [OED]. Compare toad-cheese, a poisonous fungi; toad's meat (1886), a "rustic" term for toadstool.
whitecap (n.)youdaoicibaDictYouDict
1660s, of birds, from white (adj.) + cap (n.). Attested from 1773 in reference to breaking waves, from 1818 of mushrooms, and from 1891 in reference to "one of a self-constituted band in U.S. who committed outrages under pretense of regulating public morals" [OED].
agaricoidyoudaoicibaDictYouDict
"Relating to or characteristic of an agaric. Also: belonging to the genus Agaricus or (more widely) the order Agaricales of gilled mushrooms", Early 19th cent.; earliest use found in Robert Kaye Greville (1794–1866), botanist. From agaric + -oid.