Over one's head

Meaning/Usage: When something is mentally too difficult to understand.

Explanation: To understand something, information needs to go inside the brain. This is used to say that the information failed to go inside.

"I don't know the answer to that problem. The subject is over my head."
"Don't bother explaining it to him. It's way over his head."

A: "I have to write a paper on the theory of relativity. Do you know anything about it?"
B: "I won't be able to help you. The theory of relativity is way over my head."

Other Common Sentences

"I don't understand the meaning. It might be too deep for me."
"It's beyond my comprehension, so don't bother trying to explain it."