He was a well-built man, in his forties.

well-built 是由副词 well（好地）和形容词 built（build 的过去分词）组成的复合形容词。built 来自动词 build（建造），这里比喻人的体格像精心建造的一样结实。well-built 合起来表示"体格健壮的、身材结实的"。