clench 
(klĕnch)
(klĕnch)tr.v. clenched, clench·ing, clench·es
1. To close tightly: clench one's teeth; clenched my fists in anger.
2. To grasp or grip tightly: clenched the steering wheel.
3. To clinch (a bolt, for example).
n.
1. A tight grip or grasp.
2. Something, such as a mechanical device, that clenches or holds fast.
[Middle English clenchen, from Old English -clencan (in beclencan, to hold fast).]