/peen/ , n.
1. a wedgelike, spherical, or other striking end of a hammer head opposite the face.
v.t.
2. to enlarge, straighten, or smooth with a peen.
3. to strengthen (a metal surface) by light hammering or by bombardment with steel balls or shot.
[ 1505-15; earlier pen pen (n.) in same sense (perh. Pinne peen). See PIN ]