The Collaborative International Dictionary of English v.0.48:

Pecker \Peck"er\ (p[e^]k"[~e]r), n.
   1. One who, or that which, pecks; specif., a bird that pecks
      holes in trees; a woodpecker.
      [1913 Webster]

   2. An instrument for pecking; a pick. --Garth.
      [1913 Webster]

   3. The penis; -- an obscene term for the male sexual organ.
      [Vulgar]

   Syn: penis, cock, prick, tool, peter, dick, shaft [all but
        the first considered obscene].
        [PJC]

   Flower pecker. (Zool.) See under Flower.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pecker
    n 1: obscene terms for penis [syn: cock, prick, dick,
         shaft, pecker, peter, tool, putz]
    2: bird with strong claws and a stiff tail adapted for climbing
       and a hard chisel-like bill for boring into wood for insects
       [syn: woodpecker, peckerwood, pecker]
    3: horny projecting mouth of a bird [syn: beak, bill, neb,
       nib, pecker]