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. ( INFORMAL )
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.
PHRASAL VERB : usu with brd-neg , V P that / wh , V P to n that / wh , V P