The fitness trainer looks really studly with his muscular build.

studly 是形容词，由名词 stud（种马、雄性动物）加上后缀 -ly 构成，用来形容男性"健壮有魅力的"、"充满阳刚之气的"。这个词带有非正式和幽默色彩，常用于口语中形容肌肉发达、很有男子气概的男性。