en_tn/gen/38/08.md

Onan

This is the name of one of Judah's sons. See how you translated this name in Genesis 38:4. (See: rc://en/ta/man/translate/translate-names)

Go in to your brother's wife ... he went in to

This is a euphemism. Alternate translation: "Have sexual relations with your brother's wife ... he had sexual relations with" (See: rc://en/ta/man/translate/figs-euphemism)

Do the duty of a brother-in-law to her

This refers to a custom that when the oldest brother dies before he and his wife have a son, the next oldest brother would marry and have sexual relations with the widow. When the widow gave birth to the first son, that son was considered the son of the oldest brother and he would receive the oldest brother's inheritance.