Green’s Dictionary of Slang

usu. in pl., a pectoral muscle, the development of which is popular among body-builders.

1966		L. Cohen Beautiful Losers (1970) 159: I saw you with massive lower pecs and horseshoe triceps, with bulk and definition simultaneously [OED].
1978		L. Kramer Faggots 18: What had at three feet appeared to be well-formed pecs (so important), at two feet were revealed as sagging tits.
1989		Kirk & Madsen After The Ball 314: By being terrific in bed? [...] By having pecs or thighs of death?
1994		T. Willocks Green River Rising 229: Sweat ran down his throat and into the shallow cleft between his pecs.
2002		J. Franzen Corrections 52: Denise in the photo was wearing jeans and a tank top and was all muscled shoulders and satiny pecs.
2008		R. Price Lush Life 66: Taking a step back and palming his pecs .
2009		P. Temple Truth 55: The front-seat passenger stuck his gun out [...] fired four shots, took away a big piece of Colby’s right pec, a bit of an ear.
2015		J. Stahl OG Dad 61: Some grinning douchbag driving a convertible while remobing hair from his pecs.