What is it?
In short, a wingboner is the pegasus equivilant to a human erection, with the human reproductive system replaced with the wings of a pegasus/griffion/dragon. In many images, this is usually accompanied by the short word, "pomf."
How did it get there?
In the episode Over a Barrel, before Pinkie Pie appears on stage to sing "You Gotta Share, You Gotta Care", there is a crowd shot showing Rainbow Dash with her wings closed.
After Pinkie appears on stage (in a dress and fishnet stockings) there is another crowd shot in which Dash's wings are spread. Some decided that this was rather suggestive, thus the term "Wingboner" was born.
Where is it now?
Since it's origin, the wingboner meme has become widely popular on ponychan. It has spawned hundreds of images, inspired short /oat/meal roleplays (see Ferris Wheel), fanfiction works, has been implemented into just about every canon character, and several original characters, and there have even been 3 "official" wingboner threads on /oat/meal, the first of which was pinned.
One of the wingboner meme's most memorable moments is perhaps when several users decided to make simple Ember Storm comics depicting the original character coming into, presumably, a thread or a room, and immediately getting a wingboner as the result of an image or a post. This all of course spawned off of the idea that Ember Storm had a deep, romantic connection with the main cast pony Twilight Sparkle, which was later confirmed by Ember himself. The meme originated from a few pictures by Rainy Dash as a joke.
In fact, his popularity is so high that even KYM (know your meme) made a subsection of ember storm on the wingboner article.