1580s, "lump; cluster or small, close group" (especially of shrubs or trees), from Middle English clompe "a lump" (c. 1300), from a Low German source (such as Dutch klomp "lump, mass," or Middle Low German klumpe "clog, wooden shoe"). Old English had clympre "lump, mass of metal."
"to heap or gather in clumps" (transitive), 1824, from clump (n.). Related: Clumped; clumping. Intransitive sense "to form a clump or clumps" is recorded from 1896.
"walk heavily and clumsily," 1660s, imitative, or perhaps from the notion of walking in wooden shoes (see clump (n.)). Related: Clumped; clumping.