וַיֵּ֣דַע אוֹנָ֔ן כִּ֛י לֹּ֥א ל֖וֹ יִהְיֶ֣ה הַזָּ֑רַע וְהָיָ֞ה אִם־בָּ֨א אֶל־אֵ֤שֶׁת אָחִיו֙ וְשִׁחֵ֣ת אַ֔רְצָה לְבִלְתִּ֥י נְתָן־זֶ֖רַע לְאָחִֽיו׃
俄南知道這後裔不歸他自己，所以每當他與他哥哥的妻子同房的時候，就洩在地上，免得給他哥哥留下後裔。
Rashi
ושחת ארצה AND HE DESTROYED ONTO THE GROUND — He 'threshed inside and winnowed outside' (Genesis Rabbah 85:5).
Rashbam
כי לא לו יהיה הזרע, people in those days were extremely fussy about this point.
Radak
וידע אונן, he realised this only when his father said to him that it was his task to see that his brother’s seed would be perpetuated. ושחת ארצה, he ejaculated outside her vagina, just as we explained in connection with Er.