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"insect," 1620s (earliest reference is to bedbugs), of unknown origin, probably but not certainly from or influenced by Middle English bugge "something frightening, scarecrow" (late 14c.), a meaning obsolete since the "insect" sense arose except in bugbear (1570s) and bugaboo (q.v.).
Probably connected with Scottish bogill "goblin, bugbear," or obsolete Welsh bwg "ghost, goblin" (cf. Welsh bwgwl "threat," earlier "fear," Middle Irish bocanách "supernatural being"). Some speculate that these words are from a root meaning "goat" (see buck (n.1)) and represent originally a goat-like spectre. Cf. also bogey (n.1) and German bögge, böggel-mann "goblin." Perhaps influenced in meaning by Old English -budda used in compounds for "beetle" (cf. Low German budde "louse, grub," Middle Low German buddech "thick, swollen").
In the United States bug is not confined, as in England, to the domestic pest, but is applied to all insects of the Coleoptera order, which includes what in this country are generally called beetles. [Farmer & Henley, "Dictionary of Slang and Colloquial English," 1912 abridged edition]Meaning "defect in a machine" (1889) may have been coined c.1878 by Thomas Edison (perhaps with the notion of an insect getting into the works). Meaning "person obsessed by an idea" (e.g. firebug) is from 1841, perhaps from notion of persistence. Sense of "microbe, germ" is from 1919. Bugs "crazy" is from c.1900. Bug juice as a slang name for drink is from 1869, originally "bad whiskey." The 1811 slang dictionary has bug-hunter "an upholsterer." Bug-word "word or words meant to irritate and vex" is from 1560s.
"to bulge, protrude," 1872, originally of eyes, perhaps from a humorous or dialect mispronunciation of bulge (v.). Related: Bugged; bugging. As an adjective, bug-eyed recorded from 1872; so commonly used of space creatures in mid-20c. science fiction that the initialism BEM for bug-eyed monster was current by 1953.
"to annoy, irritate," 1949, probably from bug (n.) and a reference to insect pests. Sense of "equip with a concealed microphone" is from 1919. Related: Bugged; bugging.
A true bug, specifically one having a beaklike structure that allows piercing and sucking.
An insect or similar organism, such as a centipede or an earwig.
A disease-producing microorganism, such as a flu bug.
The illness or disease so produced.
A defect or difficulty, as in a system or design.
Our Living Language : The word bug is often used to refer to tiny creatures that crawl along, such as insects and even small animals that are not insects, such as spiders and millipedes. But for scientists the word has a much narrower meaning. In the strictest terms bugs are those insects that have mouthparts adapted for piercing and sucking. The mouthparts of these bugs are contained in a beak-shaped structure. Thus scientists would classify a louse but not a beetle or a cockroach as a bug. In fact, scientists often call lice and their relatives true bugs to distinguish them better from what everyone else calls "bugs."
A generic term that describes a malfunction of undetermined origin in a computer or other electronic device.
Note: The term originated in the 1940s when the examination of a large computer revealed that an actual insect had landed on one of the circuits, shorting it out and shutting the machine down.
[Imitation of baby talk]
To protrude; bulge: Her eyes bugged out when she saw the bill
[1870s+ Dialect; fr humorous or dialectal pronunciation of bulge]