a. [ heb. koshër fit, proper. ] 1. Ceremonially clean, according to Jewish law; -- applied to food, esp. to meat of animals slaughtered according to the requirements of Jewish law. Opposed to tref. For food to be officially kosher, it must be certified fit to eat by a Rabbi, according to Jewish ritual law; as, kosher food. Hence, designating a shop, store, house, etc., where such food is sold or used; as, a kosher restaurant.
Syn. -- cosher, ritually fit, sanctioned. [ Webster 1913 Suppl. ]

2. Hence: Proper; seemly; appropriate; legitimate; as, it's not kosher to do it that way. [ colloq. ] [ PJC ]

3. Genuine. [ colloq. ] [ PJC ]

n. Kosher food; also, a kosher shop. [ Webster 1913 Suppl. ]

2. the practise of adherence to the Jewish ritual law; used mostly in the phrase
keep kosher, v. i.. [ PJC ]

keep kosher To adhere to the rules for eating only kosher food and handling it properly. [ PJC ]

v. t. [ imp. & p. p. Koshered p. pr. & vb. n. Koshering. ] To prepare in conformity with the requirements of the Jewish law, as meat. [ Webster 1913 Suppl. ]