pectoral (adj.) Look up pectoral at
1570s, "pertaining to the breast," from Latin pectoralis "of the breast," from pectus (genitive pectoris) "breast, chest," from PIE root *peg- "breast."
pectoral (n.) Look up pectoral at
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.