let him into the secret
They shared with him a secret or a subject they were dealing with.
("And the secret of the matter is, because at the time of creation He gave to man every herb yielding seed, and every tree in which is the fruit of a tree, for food, and He gave to the living creature every green herb for food" - Ramban, Bereshit 9:6)
Translation
Expression (Hebrew)
הוּבָא בְּסוֹד הָעִנְיָן
Definition (Hebrew)
שִׁתְּפוּ אוֹתוֹ בְּסוֹד אוֹ בַּנּוֹשֵׂא בּוֹ הֵם עָסְקוּ.
("וְסוֹד הָעִנְיָן, כִּי בְּעֵת הַיְּצִירָה נָתַן לָאָדָם אֶת כָּל עֵשֶׂב זֹרֵעַ זֶרַע וְאֶת כָּל הָעֵץ אֲשֶׁר בּוֹ פְרִי עֵץ לְאַכְּלָהּ, וְנָתַן לַחַיָּה אֶת כָּל יֶרֶק עֵשֶׂב לְאָכְלָה"- רַמְבָּ"ן בְּרֵאשִׁית ט ו)