Etymology
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buttermilk (n.)
1520s, from butter (n.) + milk (n.). Compare German Buttermilch, Dutch botermelk. It is what remains after the butter has been churned out.
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butterscotch (n.)

toffee-like confection, 1802, from butter (n.), which is a main ingredient; the second element uncertain; perhaps from its having been made in Scotland.

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buttercup (n.)
common name of a type of small wildflower with a yellow bloom, 1777, perhaps (OED) a merger of two older names, gold-cups and butterflower. See butter (n.) + cup (n.).
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butane (n.)
paraffin hydrocarbon, 1875, from butyl, hydrocarbon from butyric acid, a product of fermentation found in rancid butter, from Latin butyrum (see butter (n.)) + chemical suffix -ane.
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butternut (n.)
also butter-nut, 1753, nut of the white walnut, a North American tree; transferred to the tree itself from 1783, from butter (n.) + nut (n.). So called from the oil it contains.

The dye made from the tree's inner bark was yellowish-brown, and the word was used (from 1861) to describe the Southern army troops in the American Civil War, but the exact reason is debatable. Many Southern uniforms seem to have been this color; perhaps butternut dye was extensively used in homemade uniforms (but the tree's natural range is mostly in the northeastern U.S.); perhaps some of the regulation gray uniforms faded or soiled to this color; perhaps it was because butternut was a nickname for Southerners in the Midwestern states.
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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]

A truth known for ages to poets and philosophers (atomists) which modern science ponders as a possible fact.

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*teue- 
*teuə-, also *teu-, Proto-Indo-European root meaning "to swell."

It forms all or part of: butter; contumely; creosote; intumescence; intumescent; protuberance; protuberant; psychosomatic; somato-; -some (3) "body, the body;" soteriology; Tartuffe; thigh; thimble; thousand; thole (n.); thumb; tumescent; tumid; tumor; truffle; tuber; tuberculosis; tumult; tyrosine.

It is the hypothetical source of/evidence for its existence is provided by: Avestan tuma "fat;" Greek tylos "callus, lump;" Latin tumere "to swell," tumidus "swollen," tumor "a swelling;" Lithuanian tukti "to become fat;" Lithuanian taukas, Old Church Slavonic tuku, Russian tuku "fat of animals;" Old Irish ton "rump."
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*gwou- 

Proto-Indo-European root meaning "ox, bull, cow," perhaps ultimately imitative of lowing; compare Sumerian gu, Chinese ngu, ngo "ox."

It forms all or part of: beef; Boeotian; Bosphorus; boustrophedon; bovine; bugle; Bucephalus; bucolic; buffalo; bugloss; bulimia; butane; butter; butyl; butyric; cow (n.); cowbell; cowboy; cowlick; cowslip; Euboea; Gurkha; hecatomb; kine.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit gaus, Greek bous, Latin bos, Old Irish bo, Latvian guovs, Armenian gaus, Old English cu, German Kuh, Old Norse kyr, Slovak hovado "cow, ox."

In Germanic and Celtic, of females only; in most other languages, of either gender. For "cow" Latin uses bos femina or vacca, a separate word of unknown origin. Other "cow" words sometimes are from roots meaning "horn, horned," such as Lithuanian karvė, Old Church Slavonic krava.

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butterfingers (n.)
also butter-fingers, "person apt to let things fall," 1837; see butter-fingered.
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unguent (n.)
"ointment," early 15c., from Latin unguentem "ointment," from stem of unguere "to anoint or smear with ointment," from PIE root *ongw- "to salve, anoint" (source also of Sanskrit anakti "anoints, smears," Armenian aucanem "I anoint," Old Prussian anctan "butter," Old High German ancho, German anke "butter," Old Irish imb, Welsh ymenyn "butter").
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