He left his mother's lap and embraced the lap of a stranger.
In the question: He abandoned his family for the good of another family. Or: He abandoned his native land for the good of a foreign land.
("It is forbidden to sail to the Great Sea from now on for a priest who goes out into the land of the wild ram and goes out without the consent of the wise men. It is forbidden for him to shave... His brother said to her, "The man who laid his mother's lap and embraced the lap of a foreign woman, and blessed are those who were wounded, and you want to do as he goes out in it" - Yerushalmi Mo'ed Katan 3:1)
Translation
Expression (Hebrew)
הֵנִיחַ חֵיק אִמּוֹ וְחִבֵּק חֵיק נָכְרִיָּה
Definition (Hebrew)
בְּהַשְׁאָלָה: נָטַשׁ אֶת מִשְׁפַּחְתּוֹ לְטוֹבַת מִשְׁפָּחָה אַחֶרֶת. אוֹ: נָטַשׁ אֶת אֶרֶץ מוֹלַדְתּוֹ לְטוֹבַת אֶרֶץ זָרָה.
("אָסוּר לְפָרֵשׁ לַיָּם הַגָּדוֹל מֵעַתָּה כֹּהֵן שֶׁיָּצָא חוּץ לָאָרֶץ הוֹאִיל וְיָצָא שֶׁלֹּא בִּרְצוֹן חֲכָמִים יְהֵא אָסוּר לוֹ לְגַלֵּחַ...אָמַר לֵהּ אָחִיו שֶׁל אוֹתוֹ הָאִישׁ הֵנִיחַ חֵיק אִמּוֹ וְחִבֵּק חֵיק נָכְרִיָּה וּבָרוּךְ שֶׁנָּגְפוּ וְאַתְּ מְבַקֵּשׁ לַעֲשׂוֹת כַּיּוֹצֵא בּוֹ"- יְרוּשַׁלְמִי מוֹעֵד קָטָן ג, א)