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"ritually fit or pure" (especially of food), 1851, from Yiddish kosher, from Hebrew kasher "fit, proper, lawful," from base of kasher "was suitable, proper." Generalized sense of "correct, legitimate" is from 1896.
Food that is permitted according to a set of dietary restrictions found in the Old Testament. For many Jews, foods that are not kosher cannot be eaten. The term can also be used colloquially to mean anything acceptable: “I don't think it's kosher to yell at your chess opponent when he is thinking about his next move.”
Proper; as it should be; legitimate: Everything looks kosher
[1896+; fr Yiddish fr Hebrew kasher, ''fit, proper'']
("fit," or "proper"), in Judaism, the fitness of an object for ritual purposes. Though generally applied to foods that meet the requirements of the dietary laws (kashruth), kosher is also used to describe, for instance, such objects as a Torah scroll, water for ritual bathing (mikvah), and the ritual ram's horn (shofar). When applied to food, kosher is the opposite of terefah ("forbidden"); when applied to other things, it is the opposite of pasul ("unfit").