The term 'thick, messy substance' can be described by three different words: GOO (3 letters), GUNK (4 letters), and SLUDGE (6 letters). GOO is a sticky, viscous liquid that is often used to describe a thick, slimy substance. GUNK is a slang term for a thick, greasy, and often unpleasant substance. SLUDGE is a thick, semi-solid material that is created from the accumulation of sediment or other materials in a liquid. All three terms are used to describe a thick, messy substance.