Etymology
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Union Jack 
1670s, from union + jack (n.); properly a small British union flag flown as the jack of a ship, but it has long been in use as a general name for the union flag. The Union flag (1630s) was introduced to symbolize the union of the crowns of England and Scotland (in 1603) and was formed of a combination of the cross saltire of St. Andrew and the cross of St. George. The cross saltire of St. Patrick was added 1801 upon the union of parliaments of Great Britain and Ireland.
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filbert (n.)
"hazelnut," late 14c., from Anglo-French philber (late 13c.), from Norman dialect noix de filbert, in reference to St. Philbert, 7c. Frankish abbot, so called because the hazel nuts ripen near his feast day, Aug. 22 (Old Style). Weekley compares German Lambertsnuss "filbert," associated with St. Lambert (Sept. 17); also German Johannisbeere "red currant," associated with St. John's Day (June 24). The saint's name is Old High German Filu-berht, literally "very bright."
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Barnabas 
surname of Joseph the Levite of Cyprus (Acts iv.36), literally "son of exhortation," from Aramaic (Semitic) bar "son" + nabha "prophecy, exhortation." St. Barnabas' Day (colloquially St. Barnaby), June 11, in the Old Style calendar was reckoned the longest day of the year (Barnaby the Bright).
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Franciscan (n.)
1590s, "friar of the order founded in 1209 by St. Francis (Medieval Latin Franciscus) of Assisi" (1182-1226). Also as an adjective.
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unbeknownst (adj.)
1833, vulgar formation from unbeknown (1630s). No clear reason for the -st, but since 19c. this has become the dominant form.
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Chinatown (n.)

"region of a city where Chinese immigrants live," 1857, in a California context, from China + town. But from 1852 in a St. Helena context.

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Carthusian (adj.)
late 14c., from Latin Cartusianus, in reference to an austere order of monks founded 1086 by St. Bruno at Chartreux, village in Dauphiné, France.
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Elmo 

in St. Elmo's Fire "corposant," name given by seamen to the brushes and jets of electric light seen on the tips of masts and yard-arms, especially in storms, 1560s, from Italian fuoco di Sant'Elmo, named for the patron saint of Mediterranean sailors, a corruption of the name of St. Erasmus, an Italian bishop said to have been martyred in 303 who was invoked in the Mediterranean by sailors during storms.

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jellyroll (n.)
also jelly-roll, "cylindrical cake containing jelly or jam," 1873, from jelly (n.) + roll (n.). As slang for "vagina; sexual intercourse" it dates from 1914 ("St. Louis Blues").
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Sylvester 
masc. proper name, from Latin silvestris, literally "of a wood, of a forest, woody, rural, pastoral," from silva "wood, forest" (see sylvan). St. Sylvester's Day is Dec. 31.
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