n. the fruit of the oak, with a smooth nut in a rough cuplike base. acorn barnacle a multivalve marine cirriped, Balanus balanoides, living on rocks. acorn worm any marine wormlike animal of the phylum Hemichordata, having a proboscis and gill slits, and inhabiting seashores.
[ OE cern, rel. to cer ACRE, later assoc. with OAK and CORN(1) ]