Gunk is thick, sticky, unpleasant stuff that accumulates where you don't want it. When you clean out a sink drain and pull out a gross clump of hair, soap, and slime, that's gunk. The greasy buildup on a bicycle chain, the mystery substance stuck under a desk, the crusty residue inside an old paint can: all gunk.

The word perfectly captures what it describes: it sounds goopy and unpleasant, just like the stuff itself. You might find gunk clogging a garden hose, coating the inside of an old engine, or collecting in the corners of a bathtub. Sometimes the exact composition of gunk remains mysterious, which is part of what makes it gunk instead of just dirt or grease.

People use gunk for any yucky substance that's hard to identify or clean up. When something gets gunked up, it means gunk has built up enough to cause problems, like when a printer stops working because its gears are gunked up with dust and old ink. The beauty of gunk as a word is that you don't need to know exactly what the disgusting substance is. You just know you want it gone.