What is the X in X-mas?
"limited," 1830, past participle adjective from restrict; of documents, etc., "secret, not for public release" it is recorded from 1944. In U.S., restricted was a euphemism for "off-limits to Jews" (1947).
Manager: "I'm sorry, Mr. Marx, but we can't let you use the pool; this country club is restricted."
Groucho: "Well, my daughter's only half-Jewish; could she go in up to her knees?" [there are many versions and variations of this story, dating back to 1970s]