I tried not to let on that I had already guessed the answer.
If you do not let on that something is true, you do not tell anyone that it is true, and you keep it a secret. She never let on that anything was wrong I didn't let on to the staff what my conversation was He knows the culprit but is not letting on
make known to the public information that was previously known only to a few people or that was meant to be kept a secret; "The auction house would not disclose the price at which the van Gogh had sold"; "The actress won't reveal how old she is"; "bring out the truth"; "he broke the news to her"