Object used in the trade of sexual favors. Participants in trade include hungry girls who haven't eaten anything all day because of work, and potheads who are in possession of the doughnut.
A person, usually Christian, that has been baptized. When a human is baptized, their head is dipped into water, then pulled back out soon after, much like one might do to a doughtnut when dipping it into your coffee. Thus the name. This nickname also spawned many other nicknames, such as Waffle, Muffin, Bagel, and French Toast. (Look only at the definitions provided by 'Relic' to see what each means when used as a nickname.)
I won;t say I don;t like them, but doughnuts, muffins, and bagels overpopulate waffles and french toast so badly that it's just unfair.
In a match of Search and Destroy in Call of Duty, if a member of your team has 0 kills by round 3 or 4, it can be said they are dropping a doughnut. The term is often misused in round 1 or 2, where it is acceptable to not have any kills.