"A fungus with a fruiting body that resembles a mushroom, having a convex or flattened cap with gills on the underside", Late Middle English (originally denoting various bracket fungi with medicinal or other uses): from Latin agaricum, from Greek agarikon 'tree fungus'.
"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.