Butterfly
Old English, from butter + fly2; perhaps from the cream or yellow colour of common species, or from an old belief that the insects stole butter.
wiktionary
From Middle English buterflie, butturflye, boterflye, from Old English butorflēoge, buttorflēoge, buterflēoge (from butere(“butter”)), equivalent to butter + fly. Cognate with Dutch botervlieg, German Butterfliege(“butterfly”). The name may have originally been applied to butterflies of a yellowish color, and/or reflected a belief that butterflies ate milk and butter (compare German Molkendieb(“butterfly”, literally “whey thief”) and Low German Botterlicker(“butterfly”, literally “butter-licker”)), or that they excreted a butter-like substance (compare Dutch boterschijte(“butterfly”, literally “butter-shitter”)). Compare also German Schmetterling from Schmetten(“cream”), German Low German Bottervögel(“butterfly”, literally “butter-fowl”). More at butter, fly.
An alternate theory suggests that the first element may have originally been butor-(“beater”), a mutation of bēatan(“to beat”). [1]
Superseded non-native Middle English papilion(“butterfly”) borrowed from Old French papillon(“butterfly”).
etymonline
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]
