sponge

Where it comes from

Old English sponge or spunge came from Latin spongia and ultimately Greek spongia or spongos. The Greek word's earlier origin is unknown. It named both the aquatic animal and the porous material obtained from it long before synthetic cleaning sponges existed.

How the meaning developed

A traditional bath sponge is the cleaned flexible skeleton of certain marine animals, so the animal gave its name to the absorbent tool. Absorption then generated metaphors: people sponge up knowledge, while a person who lives at another's expense can be called a sponge.