"to grasp firmly," c. 1300, from Old English (be)clencan "to hold fast, make cling," causative of clingan (see cling, and compare clinch); compare stench/stink. Meaning "to set firmly together" (of fists, teeth, etc.) is from 1747 (clinch in this sense is attested from 1630s). Figurative sense of "fix or secure by a final act" is from 1670s. Related: Clenched; clenching.