What is the origin of the idiom "be in for a treat" and when to use it?

The idiomatic phrase "be in for a treat" does not have a specific historical origin. It is a common idiomatic expression that has evolved naturally in the English language over time. It is used to express anticipation or expectation of a positive and enjoyable experience. The phrase is often used to create a sense of excitement, surprise, or pleasure in the listener.