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.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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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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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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jim-jam (n.)
"knick-knack," 1640s, a reduplication of unknown origin.
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champ (v.)

1520s, "to chew noisily, crunch;" 1570s (of horses) "to bite repeatedly and impatiently," probably echoic; OED suggests a connection with jam (v.). Earlier also cham, chamb, etc. (late 14c.). To champ on (or at) the bit, as an eager horse will, is attested in the figurative sense by 1640s. Related: Champed; champing. As a noun, "act of biting repeatedly, action of champing," from c. 1600.

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jamboree (n.)

1866, "carousal, noisy drinking bout; any merrymaking," represented in England as a typical American English word, perhaps from jam (n.) on pattern of shivaree [Barnhart]. For the second element, Weekley suggests French bourree, a kind of rustic dance. Century Dictionary calls the whole thing "probably arbitrary." Klein thinks the word of Hindu origin (but he credits its introduction into English, mistakenly, to Kipling). Boy Scouts use is from 1920. It is noted earlier as a term in cribbage:

Jamboree signifies the combination of the five highest cards, as, for example, the two Bowers [jacks], Ace, King, and Queen of trumps in one hand, which entitles the holder to count sixteen points. The holder of such a hand, simply announces the fact, as no play is necessary; but should he play the hand as a Jambone, he can count only eight points, whereas he could count sixteen if he played it, or announced it as a Jamboree. ["The American Hoyle," New York, 1864]

Compare jambone "type of hand played by agreement in the card game of euchre."

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jams (n.)
1966, abstracted from pajamas (q.v.). Much earlier English picked up jam "a kind of frock for children" (1793) from Hindi jamah.
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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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jimmies (n.)
bits of candy as ice cream topping, by 1963, American English, of uncertain origin. Earlier it meant "delirium tremens" (1900) from earlier jim-jam (1885).
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