דברי הימים א ד:יד פירוש: רש״י

And Seraiah begot Joab, the leader of Gei Harashim Joab was the leader of a valley in which there were craftsmen. Now why do I call it the Valley of Craftsmen? Because they were craftsmen; i. e., those who dwelt in the Valley of the Craftsmen were craftsmen, and the valley was named after them.