pectoral (adj.)
1570s, "pertaining to the breast," from Latin pectoralis "of the breast," from pectus (genitive pectoris) "breast, chest," from PIE root *peg- "breast."
pectoral (n.)
early 15c., "ornament worn on the breast," from Middle French pectoral and directly from Latin pectorale "breastplate," noun use of neuter of adjective pectoralis (see pectoral (adj.)).

As a shortened form of pectoral muscle, attested from 1758. Slang shortening pec for this is first recorded 1966. Related: Pectorals; pecs.