Etymology
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infra dig. 
"beneath one's dignity, unbecoming to one's position in society," 1824, colloquial abbreviation of Latin infra dignitatem "beneath the dignity of." See infra- + dignity.
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infra (adv.)
"under, below, further on," from Latin infra "below, under, beneath" (see infra-). A Latin word sometimes encountered in footnotes.
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infra- 
word-forming element meaning "below, beneath," from Latin infra (adverb and preposition) "below, underneath, on the under side, beneath," also "later than; smaller than; inferior to," related to infernus "low, below," from PIE *ndher "under" (source also of Sanskrit adnah "below," Old English under "under, among;" see under). Modern popular use of it dates from the 1920s, as an opposite to super-, often in science fiction. "This use of infra- is scarcely a Latin one" [OED].
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dig (n.)

late 17c., "a tool for digging," from dig (v.). Meaning "archaeological expedition" is from 1896. Meaning "a thrust or poke" (as with an elbow) is from 1819; figurative sense of this is by 1840.

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dig (v.)

c. 1200, diggen, "to make a ditch or other excavation," a word of uncertain origin, perhaps related to dike and ditch, either via Anglo-French diguer, from Old French digue "dike" (which is ultimately from Proto-Germanic *dīk-, from PIE root *dheigw- "to stick, fix") or directly from an unrecorded Old English verb. The older native words were deolfan (see delve), grafan (see grave (v.)).

Transitive meanings "form by excavation, make by digging," also "obtain or remove by excavation" are from late 14c.; figurative sense of "discover by effort or search" is from early 15c. Meaning "to penetrate" is from mid-15c.; transitive sense of "cause to penetrate, thrust or force in" is by 1885.

In 19c. U.S. student slang it meant "study hard, give much time to study" (1827); the 20c. slang sense of "understand" is recorded by 1934 in African-American vernacular. Both probably are based on the notion of "excavate." A slightly varied sense of "appreciate" emerged by 1939. The strong past participle dug appeared in 16c. but is not etymological.

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infra-red (adj.)
also infrared, 1873, "below the red" (in the spectrum), from infra- + red (adj.1). As a noun, also from 1873.
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infrasonic (adj.)
also infra-sonic, 1920, on the model of supersonic, etc., from infra- + sonic. Or perhaps modeled on French infra-sonore.
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v.i. 
abbreviation of Latin vide infra "see below."
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fracas (n.)
1727, from French fracas "crash, sudden noise; tumult, bustle, fuss" (15c.), from Italian fracasso "uproar, crash," back-formation from fracassare "to smash, crash, break in pieces," from fra-, a shortening of Latin infra "below" (see infra-) + Italian cassare "to break," from Latin quassare "to shake" (see quash).
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inferior (adj.)
early 15c., of land, "low, lower down, lower in position," from Latin inferior "lower, farther down" (also used figuratively), comparative of inferus (adj.) "that is below or beneath," from infra "below" (see infra-). Meaning "lower in degree, rank, grade, or importance" is from 1530s; absolutely, "of low quality or rank," also from 1530s.
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