English is a dynamic language with no language academy. Neither Congress nor Parliament decides what is correct.
Most of the people posting on CR speak a “frum” dialect, which blends many Hebrew and Yiddish words. Just because the Oxford dictionary didn’t hear of a world, doesn’t mean it isn’t a kosher word. While in some countries there is an official dialect and all other dialects and usages are by definition incorrect, that isn’t the case in English. Yeshivish is no more or less proper than the Queens English or Brooklynese or the standard American dialect (spoken in most of America west of the Appalachians and north of the Mason-Dixon line, but not in the places where most orthodox Jews live).
Many times one wants to use a Hebrew or Yiddish word, since English lacks an adequate vocabulary for many Jewish concepts.