Etymology
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forecast (v.)

late 14c., "to scheme," from fore- "before" + casten in the sense of "contrive, plan, prepare" (late 14c.; see cast (v.)). Meaning "predict events" first attested late 15c. (cast (v.) "to perceive, notice" is from late 14c.). Related: Forecasting.

Whether we are to say forecast or forecasted in the past tense & participle depends on whether we regard the verb or the noun as the original from which the other is formed; ... The verb is in fact recorded 150 years earlier than the noun, & we may therefore thankfully rid ourselves of the ugly forecasted; it may be hoped that we should do so even if history were against us, but this time it is kind. [Fowler, 1926]
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forecast (n.)
early 15c., "forethought, prudence," probably from forecast (v.). Meaning "conjectured estimate of a future course" is from 1670s. A Middle English word for weather forecasting (also divination by reading signs in the clouds or weather) was aeromancy (late 14c.).
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forecasting (n.)
late 14c., verbal noun from forecast (v.).
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weather-cast (n.)
also weathercast, 1866, from weather (n.) + ending from forecast (n.).
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prognosticate (v.)

"foretell by means of present signs," early 15c., prenosticaten, a back-formation from prognostication and also from Medieval Latin prognosticatus, past participle of prognosticare"foretell," from Latin prognostica "sign to forecast weather," from neuter plural of Greek prognōstikos "foreknowing," from progignōskein (see prognosis). Related: Prognosticated; prognosticating.

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prognosis (n.)

1650s, "forecast of the probable course and termination of a case of a disease," from Late Latin prognosis, from Greek prognōsis "foreknowledge," also, in medicine, "predicted course of a disease," from stem of progignōskein "come to know beforehand," from pro- "before" (see pro-) + gignōskein "come to know" (from PIE root *gno- "to know").

An earlier form in the same sense was pronostike (early 15c.), from Medieval Latin pronosticum. The general (non-medical) sense of "a forecast of the course of events" in English is from 1706. A back-formed verb prognose is attested from 1837; the earlier verb was Middle English pronostiken (c. 1400), from Medieval Latin pronosticare.  Related: Prognosed; prognosing.

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prognostication (n.)

"foretelling or foreshadowing of future events by present signs," especially "act of making a medical prognosis," late 14c., pronasticacioun, from Old French pronosticacion (14c.) and directly from Medieval Latin *prognosticationem (nominative prognosticatio), noun of action from past-participle stem of prognosticare "foretell," from Latin prognostica "sign to forecast weather," from neuter plural of Greek prognōstikos "foreknowing," from progignōskein "come to know beforehand" (see prognosis). The -g- was restored in the English word 16c.

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forecastle (n.)
c. 1400 (mid-14c. as Anglo-French forechasteil), "short raised deck in the fore part of the ship used in warfare," from Middle English fore- "before" + Anglo-French castel "fortified tower" (see castle (n.)). In broader reference to the part of a vessel forward of the fore rigging, late 15c.; hence, generally, "section of a ship where the sailors live" (by 1840). Spelling fo'c'sle reflects sailors' pronunciation. If at the aft part of a ship, it was an afcastle.
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butterfly (n.)

common name of any lepidopterous insect active in daylight, Old English buttorfleoge, evidently butter (n.) + fly (n.), but the name is of obscure signification. Perhaps based on the old notion that the insects (or, according to Grimm, witches disguised as butterflies) consume butter or milk that is left uncovered. Or, less creatively, simply because the pale yellow color of many species' wings suggests the color of butter. Another theory connects it to the color of the insect's excrement, based on Dutch cognate boterschijte. Also see papillon.

Applied to persons from c. 1600, originally in reference to vain and gaudy attire; by 1806 in reference to transformation from early lowly state; in reference to flitting tendencies by 1873. The swimming stroke so called from 1935. As a type of mechanical nut, 1869. Butterflies "light stomach spasms caused by anxiety" is from 1908. "Who breaks a butterfly upon a wheel?" is from Pope.

The butterfly effect is a deceptively simple insight extracted from a complex modern field. As a low-profile assistant professor in MIT's department of meteorology in 1961, [Edward] Lorenz created an early computer program to simulate weather. One day he changed one of a dozen numbers representing atmospheric conditions, from .506127 to .506. That tiny alteration utterly transformed his long-term forecast, a point Lorenz amplified in his 1972 paper, "Predictability: Does the Flap of a Butterfly's Wings in Brazil Set Off a Tornado in Texas?" [Peter Dizikes, "The Meaning of the Butterfly," The Boston Globe, June 8, 2008]
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