Green’s Dictionary of Slang

the latissimus dorsi muscles.

1967		J. Rechy Numbers (1968) 61: He stands very straight, spreading his ‘lats’ like batwings.
1973		C. Gaines Stay Hungry 117: He’s got the best lats you ever saw.
1992		K. Lette Llama Parlour 173: ‘And these [...] are what I’m really workin’ on now — my lats.’ Two little wings of flesh appeared on the sides of his torso. ‘Latisimas Dorsi,’ he explained.
2001		J. Stahl Plainclothes Naked (2002) 209: He [...] could all but feel the hot metal searing through his massive lats.
2007		C. Goffard Snitch Jacket 165: What you need is some ab work [...] some military presses, quads, lats, and glutes.
2013		C. Dundas ‘Rightful King of Wrestling’ in ThugLit Nov.-Dec. [ebook] ‘[S]he came up backstage to tell me I had a nice look but needed to work on my lats’.