noun Etymology: Middle English akern, from Old English æcern; akin to Middle High German ackeran acorns collectively, Old Irish áirne sloe, Lithuanian uoga berry Date: before 12th century the nut of the oak usually seated in or surrounded by a hard woody cupule of indurated bracts
acorn woodpecker noun Date: 1981 a woodpecker ( Melanerpes formicivorus ) found from the United States Pacific coast south to Colombia that stores acorns in small holes which it digs especially in the bark of trees
acorn worm noun Date: circa 1889 any of a class (Enteropneusta) of burrowing wormlike marine animals having an acorn-shaped proboscis and classified with the hemichordates