Green’s Dictionary of Slang

diesel adj.

[diesel n. (2)]

strong, tough, muscular.

[US]L. Stavsky et al. A2Z.
[US]Eble Campus Sl. Sept.
Online Sl. Dict. [Internet] diesel adj 1. muscular. (‘Check out his body, he’s diesel.’).
[US]Eble Campus Sl. Nov. 2: diesel – very muscular: After three years of going to the gym, my brother is diesel.
Twitter 27 May [Internet] There’s woman in #N7 (what we used to call ‘a real Diesel number’: is that still OK?) has *every inch* of her neck, face, skull tattoo’d.