Kosher came into English in 1850 from Yiddish, and behind the Yiddish is Hebrew kasher, "fit, proper, lawful." Early English spellings wobbled between kosher, kasher and coshar, which is normal for a word arriving by ear from a language written in another alphabet: there is no established transliteration to copy, so writers guess.
It belongs to the Yiddish and Hebrew layer of English that also gave bagel, schlep, schmaltz, klezmer and chutzpah — mostly through American English, mostly from the late 1800s onward. Like several of them, it generalised quickly out of its religious sense: the loose meaning of correct or legitimate, as in that doesn't seem kosher, is recorded by 1896, less than fifty years after the word arrived.
kosher (adj.) "ritually fit or pure, clean, lawful, conforming to the requirements of the Talmud" (especially of food), 1850, in early use also kasher , coshar , from Yiddish kosher , from Hebrew kasher "fit, proper, lawful," from base of kasher "was suitable, proper." Generalized sense of "correct, legitimate" is from 1896. also from 1850
kosher came in through Later and wider borrowings, and its spelling follows that family's conventions.
What gives it away on the page
| schl-, schm-, chutz- | Yiddish and Hebrew, where ch is a throat sound.schlep, schmaltz, chutzpah, kosher, bagel |
Later and wider borrowings in the spelling guide · guess another word's family