וַֽתִּפְקְדִ֔י אֵ֖ת זִמַּ֣ת נְעוּרָ֑יִךְ בַּעְשׂ֤וֹת מִמִּצְרַ֙יִם֙ דַּדַּ֔יִךְ לְמַ֖עַן שְׁדֵ֥י נְעוּרָֽיִךְ׃ (ס)
Thus thou didst call to remembrance the lewdness of thy youth, when they from Egypt bruised thy breasts for the bosom of thy youth.
Read midrashic interpretation of Ezekiel 23:21: and classical rabbinic exegesis.