What is "jam nut"

Nut \Nut\ (n[u^]t), n. [OE. nute, note, AS. hnutu; akin to D. noot, G. nuss, OHG. nuz, Icel. hnot, Sw. n["o]t, Dan. n["o]d.]

1. (Bot.) The fruit of certain trees and shrubs (as of the almond, walnut, hickory, beech, filbert, etc.), consisting of a hard and indehiscent shell inclosing a kernel.

2. A perforated block (usually a small piece of metal), provided with an internal or female screw thread, used on a bolt, or screw, for tightening or holding something, or for transmitting motion. See Illust. of 1st Bolt.

3. The tumbler of a gunlock.
   --Knight.

4. (Naut.) A projection on each side of the shank of an anchor, to secure the stock in place.

5. pl. Testicles. [vulgar slang]

   Check nut, Jam nut, Lock nut, a nut which is screwed up tightly against another nut on the same bolt or screw, in order to prevent accidental unscrewing of the first nut.

   Nut buoy. See under Buoy.

   Nut coal, screened coal of a size smaller than stove coal and larger than pea coal; -- called also chestnut coal.

   Nut crab (Zo["o]l.), any leucosoid crab of the genus Ebalia as, Ebalia tuberosa of Europe.

   Nut grass (Bot.), See nut grass in the vocabulary.

   Nut lock, a device, as a metal plate bent up at the corners, to prevent a nut from becoming unscrewed, as by jarring.

   Nut pine. (Bot.) See under Pine.

   Nut rush (Bot.), a genus of cyperaceous plants ( Scleria) having a hard bony achene. Several species are found in the United States and many more in tropical regions.

   Nut tree, a tree that bears nuts.

   Nut weevil (Zo["o]l.), any species of weevils of the genus Balaninus and other allied genera, which in the larval state live in nuts.

Jam \Jam\, n. [Prob. fr. jam, v.; but cf. also Ar. jamad ice, jelly, j[=a]mid congealed, jamd congelation, ice.] A preserve of fruit boiled with sugar and water; also called jelly; as, raspberry jam; currant jam; grape jam.

Jam nut. See Check nut, under Check.

Jam weld (Forging), a butt weld. See under Butt.

A jam nut is a low profile type of nut, typically half as tall as a standard nut. It is commonly used as a type of locknut, where it is "jammed" up against a standard nut to lock the two in place. It is also used in situations where a standard nut would not fit.

The term "jam nut" can also refer to any nut that is used in the same function (even a regular-thickness nut used for the jamming purpose). Jam nuts, other types of locknuts, lock washers, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.