Etymology
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Bellatrix 

bright star in the left shoulder of Orion, from Latin bellatrix "female warrior," frequently used as an adjective, "warlike, skilled in war," fem. of bellator "to wage war," from bellum "war" (see bellicose). The Latin name, from the Alfonsine Tables (mid-13c.), very loosely translates the Arabic name for the star, Al Najid "the conqueror."

In astrology it was the natal star of all destined to great civil or military honors, and rendered women born under its influence lucky and loquacious; or as old Thomas Hood said, "Women born under this constellation shall have mighty tongues." [Richard Hinckley Allen, "Star Names and Their Meanings," 1899]
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Tartar 

mid-14c. (implied in Tartary, "the land of the Tartars"), from Medieval Latin Tartarus, from Persian Tatar, first used 13c. in reference to the hordes of Ghengis Khan (1202-1227), said to be ultimately from Tata, a name of the Mongols for themselves. Form in European languages probably influenced by Latin Tartarus "hell" (e.g. letter of St. Louis of France, 1270: "In the present danger of the Tartars either we shall push them back into the Tartarus whence they are come, or they will bring us all into heaven").

The historical word for what now are called in ethnological works Tatars. A Turkic people, their native region was east of the Caspian Sea. Ghengis' horde was a mix of Tatars, Mongols, Turks, etc. Used figuratively for "savage, rough, irascible person" (1660s). To catch a Tartar "get hold of what cannot be controlled" is recorded from 1660s; original sense not preserved, but probably from some military story similar to the old battlefield joke:

Irish soldier (shouting from within the brush): I've captured one of the enemy.
Captain: Excellent! Bring him here.
Soldier: He won't come.
Captain: Well, then, you come here.
Soldier: I would, but he won't let me.

Among the adjectival forms that have been used are Tartarian (16c.), Tartarous (Ben Jonson), Tartarean (17c.); Byron's Tartarly (1821) is a nonce-word (but a good one). Tartar sauce is attested by 1855, from French sauce tartare.

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Valentine (n.)

mid-15c., "sweetheart chosen on St. Valentine's Day," from Late Latin Valentinus, the name of two early Italian saints (from Latin valentia "strength, capacity;" see valence). Choosing a sweetheart on this day originated 14c. as a custom in English and French court circles. Meaning "letter or card sent to a sweetheart" first recorded 1824. The romantic association of the day is said to be from it being around the time when birds choose their mates.

For this was on seynt Volantynys day
Whan euery bryd cometh there to chese his make.
[Chaucer, "Parlement of Foules," c. 1381]

Probably the date was the informal first day of spring in whatever French region invented the custom (many surviving medieval calendars reckon the start of spring on the 7th or 22nd of February). No evidence connects it with the Roman Lupercalia (an 18c. theory) or to any romantic or avian quality in either of the saints. The custom of sending special cards or letters on this date flourished in England c. 1840-1870, declined around the turn of the 20th century, and revived 1920s.

To speak of the particular Customs of the English Britons, I shall begin with Valentine's Day, Feb. 14. when young Men and Maidens get their several Names writ down upon Scrolls of Paper rolled up, and lay 'em asunder, the Men drawing the Maidens Names, and these the Mens; upon which, the Men salute their chosen Valentines and present them with Gloves, &c. This Custom (which sometimes introduces a Match) is grounded upon the Instinct of Animals, which about this Time of the Year, feeling a new Heat by the approach of the Sun, begin to couple. ["The Present State of Great Britain and Ireland" London, 1723]
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