ADJ: like skinnyfat, a person whose initial physical appearance hides their true potential. "Shortstrong" describes people who are of slight stature and not broad or obviously muscular, but are capable of surprising physical poweress given one's assumed impression of their rather lithe physique.
Person A: "Simon's five foot five. He's just a little guy."
Person B: "Yeah, but last night he engaged in intercourse with that large Swiss girl in basic standing position amid a rainstorm. He's shortstrong."
Person A: "No doubt, that would have taken some serious muscle."