Why is the ninth month called September?
1911, American English, originally baseball slang; perhaps ultimately from jyng "a charm, a spell" (17c.), originally "wryneck," a bird used in witchcraft and divination, from Latin iynx "wryneck," from Greek iynx.
Most mysterious of all in the psychics of baseball is the "jinx," that peculiar "hoodoo" which affects, at times, a man, at other times a whole team. Let a man begin to think that there is a "jinx" about, and he is done for for the time being. ["Technical World Magazine," 1911]The verb is 1912 in American English, from the noun. Related: Jinxed; jinxing.
: Somebody jinxed him (1917+)
[apparently fr jynx or iynx, ''wryneck woodpecker,'' fr the use of the bird in divination]