clench (n.)

1590s, "the part of a nail that clinches," from clench (v.). Also "mode of securing a nail by hammering it back once driven." Meaning "a grasp, grip" is from 1779. Compare clinch.

clench (v.)

"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.