Etymology
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jam (v.)

"to press tightly" (trans.), 1719; "to become wedged" (intrans.), 1706, of unknown origin, perhaps a variant of Middle English cham "to bite upon something; gnash the teeth" (late 14c.; see champ (v.)). Of a malfunction in the moving parts of machinery by 1851. Sense of "cause interference in radio signals" is from 1914. Meaning "play in a jam session" is from 1935. Related: Jammed; jamming. The adverb is recorded from 1825, from the verb; jam-packed is from 1901, earlier jam-full (1830).

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jam (n.2)
"a tight pressing between two surfaces," 1806, from jam (v.). Sense of "machine blockage" is from 1890, which probably led to the colloquial meaning "predicament, tight spot," first recorded 1914. Jazz meaning "short, free improvised passage performed by the whole band" dates from 1929, and yielded jam session (1933); but this is perhaps from jam (n.1) in sense of "something sweet, something excellent."
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traffic (v.)
1540s, "to buy and sell," from traffic (n.) and preserving the original commercial sense. Related: Trafficked; trafficking; trafficker. The -k- is inserted to preserve the "k" sound of -c- before a suffix beginning in -i-, -y-, or -e- (compare picnic/picnicking, panic/panicky, shellacshellacked).
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traffic (n.)

c. 1500, "trade, commerce," from  French trafique (15c.), from Italian traffico (14c.), from trafficare "carry on trade," of uncertain origin, perhaps from a Vulgar Latin *transfricare "to rub across," from Latin trans "across" (see trans-) + fricare "to rub" (see friction), with the original sense of the Italian verb being "touch repeatedly, handle."

Or the second element may be an unexplained alteration of Latin facere "to make, do." Klein suggests ultimate derivation of the Italian word from Arabic tafriq "distribution." Meaning "people and vehicles coming and going" first recorded 1825. Traffic jam is by 1908, ousting earlier traffic block (1895). Traffic circle is from 1938.

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jam (n.1)
"fruit preserve," 1730s, probably a special use of jam (v.) "press objects close together," hence "crush fruit into a preserve."
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jim-jam (n.)
"knick-knack," 1640s, a reduplication of unknown origin.
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log-jam (n.)
also logjam, "congestion of logs on a river," by 1851, American English; see log (n.1) + jam (n.2). The figurative sense is by 1890.
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snarl (n.1)
late 14c., "a snare, noose," from snarl (v.1). Meaning "a tangle, a knot" is first attested c. 1600. Meaning "a traffic jam" is from 1933.
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monger (v.)

"to traffic in, deal in," often implying a petty or disagreeable traffic, by 1897, from monger (n.). Not considered to be from Old English mangian. Related: Mongered; mongering (1846).

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road kill (n.)

also roadkill, "animal killed by vehicular traffic," 1962, from road (n.) + kill (n.). The figurative sense is from 1992.

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