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Old English buttorfleoge, evidently butter (n.) + fly (n.), but of obscure signification. Perhaps based on the old notion that the insects (or 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. An overview of words for "butterfly" in various languages can be found here. 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 1936. Butterflies "light stomach spasms caused by anxiety" is from 1908.
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]