c. 1600, from Irish lupracan, metathesis of Old Irish luchorpan, which traditionally is explained as literally "a very small body," from lu "little, small" (from PIE root *legwh- "not heavy, having little weight") + corpan, diminutive of corp "body," from Latin corpus "body" (from PIE root *kwrep- "body, form, appearance"). However, Celtic linguistic scholarship has recently found a different explanation and connected the word to Latin Lupercalia:
New research by Simon Rodway, Michael Clarke and Jacopo Bisagni, published in the journal Cambrian Medieval Celtic Studies, traces them back to the Roman Luperci. The Luperci were bands of aristocratic youths who ran naked through ancient Rome in the festival of Lupercalia on the 15 February. In the fifth century A.D. St Augustine of Hippo compared the Luperci with the Greek werewolves who were believed to change from men into wolves by swimming through a lake in Arcadia. Two centuries later Irish scholars misunderstood Augustine. They thought he meant that the Luperci were an ancient non-human race. Because they could swim they were supposed to have survived Noah's Flood and taken refuge in Ireland. So in medieval Irish legends the leprechauns or 'little Luperci' still lived under water. The wolf connection was soon forgotten and eventually the 'little Lupercus' became the familiar land-dwelling leprechaun of modern Irish folklore and tourism. [Patrick Sims-Williams, Professor of Celtic Studies in Aberystwyth University, Wales, cited at Languagelog]
Commonly spelled lubrican in 17c. English; "Century Dictionary" (1902) has it under leprechawn. Variant leithbragan probably is Irish folk etymology, from leith "half" + brog "brogue," because the spirit was "supposed to be always employed in making or mending a single shoe."
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]