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A wingboner is a slang term used to describe a pegasus pony's "erection."
The earliest known discussion about the term wingboner started in an IRC chatroom about a fanmade comic in which Rainbow Dash is pinning Twilight Sparkle, who reacts by kissing Rainbow Dash, giving Rainbow Dash a wingboner. This led some fans to hypothesize that pegasi have an involuntary response to sexual stimulation, namely the flaring of wings. Entitled “Valentine’s day," the comic was drawn by DeviantART user TwilightFlopple.