A Tank is a noun used by RuneScape players to define looking like an idiot or a moron. It also may have a foolish homosexual meaning, such as looking like a blue summer or twat or noob.
Used by annoying noobs in Ragnarok Online that are too lazy to level themselves. Will usually take all the loot and run away once they get to the level they want. If you're lucky you might get a "thx" out of them.