Scrawny individual who has a difficult time putting on mass or gaining weight. Often used as a condescending term among weight-lifters to describe skinny, and thereby weaker, males. In the bodybuilding terminology, the antonym of ectomorph is endomorph - which is a fatter or heavier set individual who conversely has a difficult time losing weight.
Kevin: Look at that ectomorph over there Keith, he's about as skinny as a twig!
Keith: Yeah I would help him out, but I don't care. He can do whatever the F**K he wanna do!