Definition from Wiktionary, the free dictionary
Arisen from clench in XVI century.
- To clasp; to interlock. [1560s]
- To make certain; to finalize. 
- I already planned to buy the car, but the color was what really clinched it for me.
2011 October 29, Neil Johnston, “Norwich 3 - 3 Blackburn”, BBC Sport:
- Vincent Kompany was sent off after conceding a penalty that was converted by Stephen Hunt to give Wolves hope. But Adam Johnson's curling shot in stoppage time clinched the points.
- To fasten securely or permanently.
- To bend and hammer the point of (a nail) so it cannot be removed. [17th century]
- To embrace passionately.
- To hold firmly; to clench.
- Clinch the pointed spear.
- To set closely together; to close tightly.
- to clinch the teeth or the fist
- (Can we find and add a quotation of Jonathan Swift to this entry?)
to clasp, to interlock
clinch (plural clinches)
- Any of several fastenings.
- The act or process of holding fast; that which serves to hold fast; a grip or grasp.
- to get a good clinch of an antagonist, or of a weapon
- to secure anything by a clinch
- (obsolete) A pun.
- (Can we find and add a quotation of Alexander Pope to this entry?)
- (nautical) A hitch or bend by which a rope is made fast to the ring of an anchor, or the breeching of a ship's gun to the ringbolts.
- A passionate embrace.
any of several fastenings