Mouth build-up is a term used to describe the accumulation of plaque, bacteria, and other debris on the teeth and gums. The term can also refer to the buildup of tartar, which is a hard, calcified substance that forms on the teeth when plaque is not removed. The crossword solution for this term is SILT, which is a type of sedimentary material composed of small particles that are suspended in water or air. SILT is often found in rivers, lakes, and oceans, and can be a major contributor to mouth build-up.