Etymology
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session (n.)

late 14c., "periodical sitting of a court," from Old French session "act or state of sitting; assembly," from Latin sessionem (nominative sessio) "act of sitting; a seat; loitering; a session," noun of action from past participle stem of sedere "to sit" (from PIE root *sed- (1) "to sit"). Sense of "period set aside for some activity" is first recorded 1920, in bull session, probably from quarter sessions courts (see quarter (n.1)). Musical sense of "recording occasion in a studio" is from 1927.

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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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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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clambake (n.)

also clam-bake, 1835, "picnic feast consisting chiefly of a mass of clams baked on heated stones," American English, from clam (n.) + bake (n.). By 1937 in jazz slang transferred to "an enjoyable time generally," especially "jam session."

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gabfest (n.)
"session of conversation," 1895, American English colloquial, from gab + -fest.
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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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