Doughnut definition

Dough + nut, first attested in in Washington Irving's Knickerbocker’s History of New York, 1809.http://www.wordorigins.org/index.php/site/comments/doughnut/ The original doughnuts were small, nut-sized balls of fried dough (the toroidal shape, resembling a mechanical nut, was to become common in the twentieth century).

A deep-fried piece of dough or batter, commonly of a toroidal (a ring doughnut) often mixed with various sweeteners and flavourings; or flattened sphere (a filled doughnut) shape filled with jam, custard or cream.
Anything in the shape of a torus.
A peel-out or skid-mark in the shape of doughnut or toroid; a 360-degree skid.
(derogatory slang) A fat person.
A spare car tyre, usually stored in the boot, that is smaller than a full sized tyre and is only intended for temporary use.