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1.
To give a blowjob.
Guy 1: So I told her to get on her knees and smile like a donut. And she did.
Guy 2: Wow, that really works?
Guy 1: EVERY TIME!
by shedamn! August 19, 2008
 
2.
To make an "O" shape with your mouth originally coined by a man named Adam Johnson.

This can be used as a sexual innuendo toward someone of the opposite sex or to a member of the same sex to signify them to "blow you"
Smile like a donut
by smilelikeadoughnut December 15, 2010