Is wax boastful an idiomatic expression?

"Wax boastful" is not an idiomatic expression in English. It's not a commonly used phrase with a specific meaning beyond the literal meaning of the words.

Here's why:

* "Wax" usually refers to something that grows or increases, like "the moon is waxing."

* "Boastful" means to show off or brag.

Combining the two words doesn't create a commonly understood idiom.

If you want to express the idea of someone becoming increasingly boastful, you could use phrases like:

* "Become increasingly arrogant"

* "Grow more and more full of themselves"

* "Their ego inflated"

* "They started bragging more and more"

Remember, using idioms correctly is important for clear communication.