nut

n.

1. A dry fruit

   seed, kernel, stone, achene, caryopsis, utricle; see also fruit 1.

   Common nuts include: peanut, acorn, beechnut, hazelnut, black walnut, English walnut, almond, nutmeg, pecan, filbert, coconut, pistachio, cashew, hognut, chestnut, water chestnut, butternut, kola; hickory, swamp hickory, pine, Brazil, macadamia, groundnut, ginkgo, quandong, betel nut.

2. A threaded metal block

   bur, lock nut, cap, ratchet nut, bolt nut, screw nut, jam nut; see also bolt 1.

3. *An eccentric or insane person

   eccentric, fanatic, maniac; see zealot.

4. *The head

   crown, skull, brain; see cranium, head 1.

hard (<strong><em>or</em> </strong>tough) nut to crack

off one's nut*