The Collaborative International Dictionary of English v.0.48:

Peen \Peen\ (p[=e]n), 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.]
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Peen \Peen\, v. t.
   To draw, bend, or straighten, as metal, by blows with the
   peen of a hammer or sledge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

peen
    n 1: the part of a hammerhead opposite the flat striking surface
         (may have various shapes)