Yours, Etc.: Origins and Uses of 8 Sign-Offs
1560s, "constructed, erected," past participle adjective from build (v.). Meaning "physically well-developed" is by 1940s (well-built in reference to a woman is from 1871); Built-in (adj.) is from 1898.
Physically well-developed, esp in a sexually attractive way; hunky, stacked: She wasn't especially smart, but she was built