Kosher means prepared according to Jewish dietary law, or above board. Kosher food, a kosher kitchen, not entirely kosher.
The rules cover which animals may be eaten, how they are killed, and the separation of meat from dairy — which is why a kosher kitchen has two sets of everything. The informal sense, meaning legitimate, came into general English from Yiddish.
1adjectivekosherPrepared according to Jewish dietary law.
“The restaurant serves kosher food, so there's no pork on the menu.”
2adjectivelegitimateLegitimate and above board.
“The deal sounds a bit suspicious; are you sure it's kosher?”
kosher is a quality of fitness.
Yiddish kosher comes from Hebrew kasher, fit or proper.
Hebrew
כָּשֵׁר
Yiddish
כּשר
English, today
kosher
Yiddish כּשר, from Hebrew כָּשֵׁר.
Etymology from Wiktionary, CC BY-SA