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).

jam (n.1)

"fruit preserve," 1730s, probably a special use of jam (v.) "press objects close together," hence "crush fruit into a preserve."

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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Definitions of jam
1
jam (v.)
press tightly together or cram;
Synonyms: throng / mob / pack / pile
jam (v.)
push down forcibly;
The driver jammed the brake pedal to the floor
jam (v.)
crush or bruise;
jam a toe
Synonyms: crush
jam (v.)
interfere with or prevent the reception of signals;
Synonyms: block
jam (v.)
get stuck and immobilized;
the mechanism jammed
jam (v.)
crowd or pack to capacity;
Synonyms: jampack / ram / chock up / cram / wad
jam (v.)
block passage through;
Synonyms: obstruct / obturate / impede / occlude / block / close up
2
jam (n.)
preserve of crushed fruit;
jam (n.)
informal terms for a difficult situation;
Synonyms: fix / hole / mess / muddle / pickle / kettle of fish
jam (n.)
a dense crowd of people;
Synonyms: crush / press
jam (n.)
deliberate radiation or reflection of electromagnetic energy for the purpose of disrupting enemy use of electronic devices or systems;
Synonyms: jamming / electronic jamming
From wordnet.princeton.edu