Though I haven't played Skyrim the Elder Scrolls, I know the origin of this rapidly spreading internet meme. It comes from the fact, that about every guard (which apparantly is a very popular type of NPC in Skyrim), tells you that he "used to be an adventurer like you, but then took an arrow to the knee" and cannot go around the country adventuring anymore as an effect of that injury. Due to this very noticeable and overused quote in the game, people all over the internet are using this modified quote to comment on something they supposedly used to do but don't do anymore. There does not have to be any logical connection between the two.
Example:
I used to play Twilight Princess, but then I took an arrow to the knee.
Type that thing on youtube to get some more examples.