Genesis 38:9

“And Onan knew that the seed should not be his, etc.] Should not be called a son of his, but a son of his brother Er; this is to be understood only of the firstborn; all the rest of the children born afterwards were reckoned the children of the real parent of them; this shows this was a custom in use in those times, and well known, and was not a peculiar case: and it came to pass, when he went in unto his brother’s wife; to cohabit with her, as man and wife, he having married her according to his father’s direction: that he spilled [it] on the ground, lest he should give his seed to his brother: lest his brother’s wife he had married should conceive by him, and bear a son that should be…”

“- The Family of Judah 1. עדלם ‛ǎdûllâm, ‘Adullam, “righteousness.” חירה chı̂yrâh Chirah, “nobility?” 2. שׁוּע shûa‛, Shua‘, “luck, riches, cry.” 3. ער ‛êr, ‘Er, “watching.” 4. אונן 'ônân, Onan, “strong.” 5. שׁלה shēlâh, Shelah, “request? rest.” כזיב kezı̂yb Kezib, “falsehood.” 6. תמר tāmār, Tamar, “palm.” 12. תמנה tı̂mnâh, Timnah, “counted or assigned.” 14. עינים 'êynayı̂m, ‘Enaim, “two fountains.” 29. פרץ perets, Perets, “breach.” This strange narrative is an episode in the history of Joseph; but an integral part of the “generations” of Jacob. It is loosely dated with the phrase “at that time.” This does not indicate a sequel to the preceding record, the proper phrase for…”

“38:9-10 Onan married Tamar, but was not willing to provide an heir for his brother. He would have sex with Tamar but not fulfill his responsibility to his dead brother (38:8).”