O.E. byldan "construct a house," verb form of bold "house," from P.Gmc. *buthlam, from PIE base *bhu- "dwell." Rare in O.E., in M.E. it won out over more common O.E. timbran. Modern spelling is unexplained. Building "a structure" is from c.1300. Built-in is from 1898; well-built in reference to a woman is from 1871.