Origin:
A phrase that developed on the RuneScape Item Discussion Forum back in December 2009. The phrase was started when the user HomerSympson created a thread asking users what they wanted for Christmas that year.

The first user responded with "i want u leg lol". The line was so random that everyone thought it was extremely funny. So a user named Oldcouch copied and responded with "i want u leg lol," then the user Proton responded with the same, and it continued for several pages before being locked by a forum moderator for spam.

Since that time, the phrase has been credited with the title of the first Item Discussion forum meme.

Usage:
The correct use of the phrase is whenever a "want" applies to the conversation.

However, it can really be used whenever, especially when a topic is completely random or useless and the user has nothing to contribute.