1640s, "course, direction," from Latin ductus "a leading, a conduit pipe," noun use of past participle of ducere "to lead" (from PIE root deuk- "to lead"). Anatomical sense "vessel of an animal body by which blood, lymph, etc., are conveyed" is from 1660s. Meaning "conduit, channel" is 1713; that of "air tube in a structure" is from 1884.