The Collaborative International Dictionary of English v.0.48:

Pigeon-breasted \Pi"geon-breast`ed\, a.
   Having a breast like a pigeon, -- the sternum being so
   prominent as to constitute a deformity; chicken-breasted.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pigeon-breasted
    adj 1: having a chest deformity marked by a projecting
           breastbone caused by infantile rickets [syn: pigeon-
           breasted, chicken-breasted]