1560s, "clasp, interlock," especially with a bent nail, variant of clench. The sense of "settle decisively" is first recorded 1716, from the notion of "clinching" the point of a nail to keep it fast. Boxing sense is from 1860. Related: Clinched; clinching.

1620s, "method of fastening," from clinch, v.. Meaning "a fastening by bent nail" is from 1650s. In pugilism, from 1875.