וַיֵּ֣דַע אוֹנָ֔ן כִּ֛י לֹּ֥א ל֖וֹ יִהְיֶ֣ה הַזָּ֑רַע וְהָיָ֞ה אִם־בָּ֨א אֶל־אֵ֤שֶׁת אָחִיו֙ וְשִׁחֵ֣ת אַ֔רְצָה לְבִלְתִּ֥י נְתָן־זֶ֖רַע לְאָחִֽיו׃
오난이 그 씨가 자기 것이 되지 않을 줄 알므로 형수에게 들어갔을 때에 형에게 아들을 얻게 아니하려고 땅에 설정하매
Rashi
ושחת ארצה AND HE DESTROYED ONTO THE GROUND — He 'threshed inside and winnowed outside' (Genesis Rabbah 85:5).
Chizkuni
וידע אונן, “Onan realised;” [the author’s problem with this expression is that the term ידע, normally refers to something in the past which was known as a fact. Ed.] The author quotes הידוע נדע, “how were we to know,” in Genesis 43,7 as proof that the term can also be used here.
Rashbam
כי לא לו יהיה הזרע, people in those days were extremely fussy about this point.