Origin and history of vein

c. 1300, "a blood vessel," in anatomy, a vein as distinguished by function from an artery, from Old French veine "vein, artery, pulse" (12c.), from Latin vena "a blood vessel," also "a water course, a vein of metal, a person's natural ability or interest," a word of unknown origin.

The mining sense of "linear deposit of metal or ore" is attested in English from late 14c. (Greek phleps "vein" also had the same secondary sense). It also was used in Middle English of channels or streams of underground water. In reference to a streak or stripe of a different color or shade in marble, etc. by 1640s.

The figurative sense of "strain or intermixture" (of some quality) is recorded from 1560s; that of "a humor or mood, natural tendency" is recorded by 1570s, perhaps c. 1400.

In Middle English also generally, "blood," hence, like heart, as the inmost seat of a feeling. Wycliffe (1382) has the veines of his grucchinge for "the essence of his complaint" (Job iv.12); all the veins meant "in every part of the body," and the priest in Chaucer's Canon's Yeoman's Tale was glad in every vein.