Sponge

A sponge in common Avatarian usage is a character who gains experience points in a group while contributing the absolute minimum to a group. This most often happens when a character follows the tank, but makes no additional attacks other than the basic melee attacks of their class. Typically silent during a run, a sponging character may go afk without warning, may be slow when following instructions from the group leader and will generally be inattentive to the goings on.

This is not to say that a sponge cannot be an effective group member, particularly if they are well equipped, but merely that they could be more effective if they were paying attention!

To many, labelling a character as a sponge is a form of insult and a player who has a reputation for sponging may find getting groups harder.

From HELP SPONGE.