let in on

US /let ɪn ɑːn/
UK /let ɪn ɑːn/
"let in on" picture
1.

einweihen in, teilhaben lassen an

to allow someone to share a secret or to know something that is not known by many people

:
They finally let me in on their plans for the surprise party.
Sie ließen mich endlich an ihren Plänen für die Überraschungsparty teilhaben.
She refused to let him in on the secret.
Sie weigerte sich, ihn in das Geheimnis einzuweihen.