The Collaborative International Dictionary of English v.0.48:

Knife \Knife\ (n[imac]f), n.; pl. Knives (n[imac]vz). [OE.
   knif, AS. cn[imac]f; akin to D. knijf, Icel. kn[imac]fr, Sw.
   knif, Dan. kniv.]
   1. An instrument consisting of a thin blade, usually of steel
      and having a sharp edge for cutting, fastened to a handle,
      but of many different forms and names for different uses;
      as, table knife, drawing knife, putty knife, pallet knife,
      pocketknife, penknife, chopping knife, etc..
      [1913 Webster]

   2. A sword or dagger.
      [1913 Webster]

            The coward conquest of a wretch's knife. --Shak.
      [1913 Webster]

   Knife grass (Bot.) a tropical American sedge (Scleria
      latifolia), having leaves with a very sharp and hard
      edge, like a knife.

   War to the knife, mortal combat; a conflict carried to the
      last extremity.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Knife \Knife\, v. t. [imp. & p. p. Knifed; p. pr. & vb. n.
   Knifing.]
   1. (Hort.) To prune with the knife.
      [1913 Webster]

   2. To cut or stab with a knife. [Low]
      [1913 Webster]

   3. Fig.: To stab in the back; to try to defeat by underhand
      means, esp. in politics; to vote or work secretly against
      (a candidate of one's own party). [Slang, U. S.]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

knife
    n 1: edge tool used as a cutting instrument; has a pointed blade
         with a sharp edge and a handle
    2: a weapon with a handle and blade with a sharp point
    3: any long thin projection that is transient; "tongues of flame
       licked at the walls"; "rifles exploded quick knives of fire
       into the dark" [syn: tongue, knife]
    v 1: use a knife on; "The victim was knifed to death" [syn:
         knife, stab]