The afternoon before Yom Kipper started I saw a colleague at work and we exchanged holiday wishes. He said 'tsom kal' to which the jerky pedant inside of me insisted on responding that "tsom kal [a light fast] is just a calque from the Yiddish of 'hot a laykhtn fast'."
He happens to live with another Yiddish pedant, so he just smiled and waved. But it was true. And not just 'tsom kal.' Not content to scold in person, I took the matter to Twitter. 'Most of modern Jewish culture is just a secret calque from Yiddish. ADMIT IT.' And while I got a little pushback on that, I also got a great reference from a Twitter friend, perfectly on point.
After yontev my brilliant Twitter friend Shlomo Kay sent me this. It's a blog post called Origins of the Phrase Hag Sameah and it's a wonderful translation and summary of a Hebrew language post on the blog of the Academy of the Hebrew Language. The thrust of the post is that the phrase hag sameah (or khag sameakh) is fairly modern, only coming into wide use at the turn of the 20th century. And it is most definitely a calque (a word for word carry over) from the Yiddish gut yontev or a freylikhn yontev.
I often talk about the way that Yiddish language and culture is erased and delegitimized-- and delegitimized by its erasure-- and this is a pretty great example.
And here's some more secret Yiddish calques off the top of my head:
gut yontev ----> khag sameakh
hot a laykhtn fast------> tsom kal
gut shabes-----> shabbat shalom
in a guter sho-----> b'sha'a tovah
Please add more in the comments!