וַיֹּ֤אמֶר יְהוּדָה֙ לְאוֹנָ֔ן בֹּ֛א אֶל־אֵ֥שֶׁת אָחִ֖יךָ וְיַבֵּ֣ם אֹתָ֑הּ וְהָקֵ֥ם זֶ֖רַע לְאָחִֽיךָ׃
And Judah said unto Onan: ‘Go in unto thy brother’s wife, and perform the duty of a husband’s brother unto her, and raise up seed to thy brother.’
Read kabbalistic (mystical) interpretation of Genesis 38:8: and classical mystical sources.