Etymology
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April fool (n.)
1680s; April-gowk (from Old Norse gaukr "a cuckoo") is a northern variant. April Fool's Day customs of sending people on false errands seem to have come to England from France late 17c.; originally All Fool's Day (1712). In Cumberland, Westmorland and northern parts of Lancashire and Yorkshire, however, May 1 was the day for hoaxing, and the fool was a May gosling. That custom was first attested 1791.
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absolute zero (n.)
"lowest possible temperature which the nature of heat admits" (determined to be –273 centigrade, –458 Fahrenheit), the idea dates back to 1702 and its general value was guessed to within a few degrees soon thereafter, but not precisely discovered until Lord Kelvin's work in 1848. It was known by many names, such as infinite cold, absolute cold, natural zero of temperature; the term absolute zero was among them by 1806.
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Waldorf salad 
1911, from Waldorf-Astoria Hotel in New York, where it first was served.
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birthday suit (n.)
first attested 1730s, but probably much older. The notion is the suit of clothes one was born in, i.e., no clothes at all. Compare Middle English mother naked "naked as the day one was born;" Middle Dutch moeder naect, German mutternackt.
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Good Friday (n.)
the Friday before Easter, c. 1300, from good (adj.) in Middle English sense of "holy, sacred," especially of holy days or seasons observed by the church; the word also was applied to Christmas and Shrove Tuesday. Good Twelfthe Dai (c. 1500) was Epiphany (the twelfth day after Christmas).
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bush league (adj.)
"mean, petty, unprofessional," 1906, from baseball slang for the small-town baseball clubs below the minor league where talent was developed (by 1903), from bush (n.) in the adjectival slang sense of "rural, provincial," which originally was simple description, not a value judgment.
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white elephant (n.)
"burdensome charge, inconvenient thing that one does not know how to get rid of," 1851, supposedly from the practice of the King of Siam of presenting one of the sacred albino elephants to a courtier who had fallen from favor; the gift was a great honor, but the proper upkeep of one was ruinously expensive.
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sub rosa (adv.)
"privately, secretly," Latin, literally "under the rose," which was regarded as a symbol of secrecy.
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guest star (n.)
1914 in the entertainment sense; earlier the phrase was used literally, of novae. As a verb, by 1942.
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red cross (n.)

early 15c. as the national emblem of England (St. George's Cross), also the badge of the Order of the Temple. Hence red-cross knight, one bearing such a marking on shield or crest. In 17c., a red cross was the mark placed on the doors of London houses infected with the plague. The red cross was adopted as a symbol of ambulance service in 1864 by the Geneva Conference, and the Red Cross Society (later also, in Muslim lands, Red Crescent) philanthropic organization was founded to carry out the views of the 1864 conference as well as other works of relief.

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