jamyoudaoicibaDictYouDict[jam 词源字典]
jam: [18] The verb jam, meaning ‘press tightly together’, first appears in the early 18th century (the earliest-known unequivocal example of its transitive use is in Daniel Defoe’s Robinson Crusoe 1719: ‘The ship stuck fast, jaum’d in between two rocks’). It is not known where it came from, but it is generally assumed to be imitative or symbolic in some way of the effort of pushing.

Equally mysterious are the origins of jam the sweet substance spread on bread, which appeared around the same time. Contemporary etymologists were nonplussed (Nathan Bailey had a stab in the 1730s: ‘prob. of J’aime, i.e. I love it; as Children used to say in French formerly, when they liked any Thing’; but Dr Johnson in 1755 confessed ‘I know not whence derived’); and even today the best guess that can be made is that the word refers to the ‘jamming’ or crushing of fruit into jars.
[jam etymology, jam origin, 英语词源]
jam (v.)youdaoicibaDictYouDict
"to press tightly," also "to become wedged," 1706, of unknown origin, perhaps a variant of champ (v.). Of a malfunction in the moving parts of machinery, by 1851. Sense of "cause interference in radio signals" is from 1914. Related: Jammed; jamming. The adverb is recorded from 1825, from the verb.
jam (n.1)youdaoicibaDictYouDict
"fruit preserve," 1730s, probably a special use of jam (v.) with a sense of "crush fruit into a preserve."
jam (n.2)youdaoicibaDictYouDict
"a tight pressing between two surfaces," 1806, from jam (v.). 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." Sense of "machine blockage" is from 1890, which probably led to the colloquial meaning "predicament, tight spot," first recorded 1914.