Leeroy Jenkins is a character in a famous video that dates back to original World of Warcraft. You can still see the video on many sites. The video has actually appeared in a question on the quiz show Jeopardy.
In the video, Leeroy and his fellow guildmates are planning their attack on the Rookery in Upper Blackrock Spire over voice chat. They are not quite done with their planning when Leeroy (apparently not paying attention even though he is mentioned by name during the planning) gets up and rushes into the Rookery yelling "Leeeeroooooooy Jenkins!". The group basically wipes with cries of confusion and dismay over voice chat. After everyone dies, Leeroy inexplicably mutters, "At least I have chicken."