Source : Webster's Revised Unabridged Dictionary (1913)

Peen \Peen\, n. [Cf. G. pinne pane of a hammer.]
   (a) A round-edged, or hemispherical, end to the head of a
       hammer or sledge, used to stretch or bend metal by
       indentation.
   (b) The sharp-edged end of the head of a mason's hammer.
       [Spelt also {pane}, {pein}, and {piend}.]

Peen \Peen\, v. t.
   To draw, bend, or straighten, as metal, by blows with the
   peen of a hammer or sledge.

Source : WordNet®

peen
     n : the part of a hammerhead opposite the flat striking surface
         (may have various shapes)