To secure or settle something definitively, or to hold someone tightly in an embrace. In boxing, to hold an opponent to prevent effective punching.
From Old English clencan meaning 'to make cling'. Related to 'clench' and 'cling'. Originally meant to fasten or make secure, like clinching a nail by bending its point. The embracing sense developed from the idea of holding tightly.
In carpentry, you 'clinch' a nail by hammering its protruding end over to secure it permanently - this is why 'clinching a deal' means making it permanent and secure. The boxing term comes from fighters literally clinging to each other to avoid punches, showing how one word can capture both the physical and metaphorical meanings of 'holding tight'.
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