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1640s, "course, direction," from Latin ductus "a leading," past participle of ducere "to lead" (see duke (n.)). Anatomical sense is from 1660s. Meaning "conduit, channel" is 1713; that of "air tube in a structure" is from 1884.
A tubular bodily canal or passage, especially one for carrying a glandular secretion such as bile.