This is one of those phrases that started off as incredibly rude, and through sheer repetition (and dropping one key word) turned into a compliment. See, the original expression, from early 1900s slang, was built like a brick shithouse.

At first it was straightforward – it was a dude who didn’t have much going for him, except by god he was muscular. Then someone got the idea of using it as a backhanded compliment for a woman … then someone left out the “shit” part … pretty soon people didn’t realize that it was insulting at all, and it just became an honest compliment for beautifully thick women.
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