1570s, "of or pertaining to the breast or chest," from Latin pectoralis "of the breast," from pectus (genitive pectoris) "breast, chest," a word of unknown origin. De Vaan considers Old Irish ucht "breast, chest" as "a likely cognate, if it reflects earler *pektu-." Pectoral muscle is attested from 1610s.
early 15c., "ornament worn on the breast," from Old French pectoral and directly from Latin pectorale "breastplate," noun use of neuter of adjective pectoralis (see pectoral (adj.)). Related: Pectorally. As a shortened form of pectoral muscle, attested from 1758. Slang shortening pec for this is first recorded 1966. Related: Pectorals; pecs.