look up any word, like bukkake:
 
1.
A completely forgettable message board user. Somewhere there is most likely a factory churning them out by the thousands. You'll know you're in the presence of a forum rat when you read a post and get the sudden strong feeling that you've read the exact same post before. Main characteristics of a forum rat:

-Infrequent or nonexistent use of capital letters and/or punctuation.
-Bizarre behavior and use of the English language, which is often described as either ghetto, otaku, forum female, fanboy, or even l33t d00d, or a combination of the above.
-Avatar consisting of an anime or video game character, animated sprites from Metal Slug or Final Fantasy, or the forum default Homer Simpson picture.
-Username consisting of at least five numbers or underscores.
-Huge, ugly Photoshop abortion of a signature which may stretch the page frames.
-Terrible or nonexistent lack of personality
-Completely worthless, typical, or irrelevant opinion on anything.

Most forum rats are preteen kids, and often either a fanboy, wigger, jock. Though they come and go, no board is safe from them unless the admin only lets his/her friends register.
"Where have all the interesting members run off to? This place is just clogged with forum rats now..."
by RealGTX March 31, 2005
 
2.
Similar to a troll, a forum rat is someone who has spent so much time on an internet forum site, such as Gametrailers or Gamespot, that they start to use the usual forum-like mannerisms in real life. Whether intentional or not, this is not a good thing. It could have originated from term mallrat.
Person A: "Dude, I totally caught this 20 pound marlin out near Cabo San Lucas last weekend"

Forum Rat: "Pics or fake."
by Vidableek October 21, 2008