grout (n.)

"thin, fluid mortar" used in joints of masonry and brickwork, 1580s, extended from sense "coarse porridge," perhaps from Old English gruta (plural) "coarse meal," from Proto-Germanic *grut-, from PIE root *ghreu- "to rub, grind" (see grit (n.)). As a verb from 1838. Related: grouted; grouting.