lexicographile
A person who is obsessed with defining new words, possibly on Urban Dictionary. A lexicographile may or may not derive sexual enjoyment from said activity. Combination of 'lexicography' (the art of dictionary-writing), and the Greek suffix '-philia' (love of).
You see that guy typing away on his computer? He's a total creep, defining words online all the time. I think he has a bad case of lexicographilia. He's a lexicographile.