be out of (something)
To no longer have a supply of something. Would you mind going to the store? We're out of milk. How can you be out of eggs? You're a diner that specializes in all-day breakfast!
out of, be
Be lacking, as in We're out of sugar and coffee. Shakespeare used this idiom in Henry V (3:7): "These English are ... out of beef." [Late 1500s] Also see run out of.