The word sponge has several meanings:

1. A soft, porous object used for cleaning or soaking up liquids. Bath sponges and kitchen sponges work by trapping water in countless tiny holes throughout their structure. Natural sponges are actually the skeletons of simple sea animals, though most sponges today are made from synthetic materials. When you sponge something off, you're wiping it clean with a sponge.
2. A person who takes advantage of others' generosity without giving anything back. If your friend always borrows your pencils, eats your snacks, and asks for help with homework but never reciprocates, you might say they're sponging off you. Someone who constantly relies on others for money, food, or favors without contributing might be called a sponge or a freeloader.
3. To absorb or soak up like a sponge does. A good student might sponge up knowledge in an interesting class, eagerly absorbing every lesson. Dry soil sponges up rainwater after a storm. When you use sponge this way, you're comparing how something absorbs ideas or liquids to how a sponge soaks up water.