Old English clott "a round mass, lump," from Proto-Germanic *klutto- (source also of Dutch kloot "ball," Danish klods "a block, lump," German Klotz "lump, block"); probably related to or confused with cleat and clod (q.v.). Of blood from 1610s,
"to form in a coagulated mass," early 15c., from clot (n.). Of fluids (especially blood) from 1590s. Related: Clotted; clotting. Clotted cream (1799) originally was clouted cream (1540s).