Whenever one goes to a restaurant in America, one is very likely to hear one or another server (esp. what used to be called a “waitress”) say to the diner “Enjoy!” after bringing a dish that the diner ordered to the table. This happens regularly to Y-H-B at the restaurants he frequents in Dorset and Manchester, Vermont, where the servers tend to be females in their 30s. Little do these women realize that Enjoy! is actually a Yiddishism.
Backing up a bit by way of explanation, first one should realize that there is a general trend in English of using transitive verbs, including reflexives, absolutely, which is to say using reflexive verbs without the reflexive postposition yourself, etc.. According to this view, Enjoy! is a truncated version of original Enjoy yourself/yourselves!
Enjoy! is a Yiddishism that probably originated in immigrant New York speech (as heard, for instance, on the 1950s television program The Goldbergs). As with much of Yiddish, this particular locution’s form derives from Russian grammar, which has the reflexive particle -sja/-es’ attached to the verbal base, so that the Russian equivalent of Enjoy! would be наслаждайтесь! (naslazhdajtes’!).
Like all the other Yiddishisms in current American English, this one is used by speakers (both Jews and gentiles) without the slightest awareness of its origin.