Source : Webster's Revised Unabridged Dictionary (1913)

Clincher \Clinch"er\, n.
   1. One who, or that which, clinches; that which holds fast.
      --Pope.

   2. That which ends a dispute or controversy; a decisive
      argument.

Source : WordNet®

clincher
     n 1: an argument that is conclusive [syn: {determiner}, {determining
          factor}]
     2: a point or fact or remark that settles something
        conclusively [syn: {decisive factor}]
     3: a tool used to clinch nails or bolts or rivets