The Collaborative International Dictionary of English v.0.48:

pecs \pecs\ (p[e^]ks), n. pl. (Anat.) [contraction from pectoral
   muscles.]
   The pectoral muscles; -- a contraction used by body-building
   and health enthusiasts. Used similarly to abs and delts.
   [informal]
   [PJC]

WordNet (r) 3.0 (2006):

pecs
    n 1: either of two large muscles of the chest [syn: pectoral,
         pectoral muscle, pectoralis, musculus pectoralis,
         pecs]