old style of wrench with a jaw adjustable by a screw mechanism on the handle, 1841, from monkey (n.) + wrench (n.). Monkey was used in 19c. especially by sailors, as a modifier for various types of small equipment made for specific work (monkey-block, monkey-boat, monkey-spar, etc.), and the same notion probably is behind the name of the tool. The figurative sense of "something that obstructs operations" is from the notion of one getting jammed in the gears of machinery (compare English spanner in the works).
"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).
for historical evolution, see V. Used punningly for you by 1588 ["Love's Labour's Lost," V.i.60], not long after the pronunciation shift that made the vowel a homonym of the pronoun. As a simple shorthand (without intentional word-play), it is recorded from 1862. Common in business abbreviations since 1923 (such as U-Haul, attested from 1951).
The substitution of Middle English -o- for Old English -u- before -m-, -n-, or -r- was a French scribal habit before minims to avoid misreading the letters in the old style handwriting, which jammed them together. The practice transformed some, come, monk, tongue, worm.
elementary intransitive verb of motion, Old English cuman "to move with the purpose of reaching, or so as to reach, some point; to arrive by movement or progression;" also "move into view, appear, become perceptible; come to oneself, recover; arrive; assemble" (class IV strong verb; past tense cuom, com, past participle cumen), from Proto-Germanic *kwem- (source also of Old Saxon cuman, Old Frisian kuma, Middle Dutch comen, Dutch komen, Old High German queman, German kommen, Old Norse koma, Gothic qiman), from PIE root *gwa- "to go, come."
The substitution of Middle English -o- for Old English -u- was a scribal habit before minims to avoid misreading the letters in the old style handwriting, which jammed them together (see U). Modern past tense form came is Middle English, probably from Old Norse kvam, replacing Old English cuom.
Meaning "to happen, occur" is from early 12c. (come to pass "happen, occur" is from 1520s). As an invitation to action, c. 1300; as a call or appeal to a person (often in expanded forms: "come, come," "come, now"), mid-14c. Come again? as an off-hand way of asking "what did you say?" is attested by 1884. For sexual senses, see cum.
Remarkably productive with prepositions (NTC's "Dictionary of Phrasal Verbs" lists 198 combinations); consider the varied senses in come to "regain consciousness," come over "possess" (as an emotion), come at "attack," come on (interj.) "be serious," and come off "occur, have some level of success" (1864). Among other common expressions are:
To come down with "become ill with" (a disease), 1895; come in, of a radio operator, "begin speaking," 1958; come on "advance in growth or development," c. 1600; come out, of a young woman, "make a formal entry into society," 1782; come round "return to a normal state or better condition," 1841; come through "act as desired or expected," 1914; come up "arise as a subject of attention," 1844; come up with "produce, present," 1934.
To have it coming "deserve what one suffers" is from 1904. To come right down to it "get to fundamental facts" is from 1875.