Genesis 38:14 - Playing the Harlot

And she put her widow's garments off from her, and covered her with a vail, and wrapped herself, and sat in an open place, which is by the way to Timnath; for she saw that Shelah was grown, and she was not given unto him to wife.

she put her widow's garments off from her, and covered her with a vail, and wrapped herself, and sat in an open place, which is by the way to Timnath - Prostitutes often dress seductively and spend most of their time in the streets and in public places soliciting their body indiscriminately for money (Proverbs 7:12). Avoid them at all cost, being careful not to be ensnared by their seductive ways.

Genesis 38:13 - Timnath or Timnah?

And it was told Tamar, saying, Behold thy father in law goeth up to Timnath to shear his sheep.

Timnath was not the Timnath of Sampson (Judges 14:1), which is on lower ground than Adullam. Most likely, it is the Timnah of Joshua 15:10, 57, about 10 mile west of Bethlehem.

Genesis 38:11 - Back Home: Used, Childless & Widowed

Then said Judah to Tamar his daughter in law, Remain a widow at thy father's house, till Shelah my son be grown: for he said, Lest peradventure he die also, as his brethren did. And Tamar went and dwelt in her father's house

Tamar went and dwelt in her father's house as this was the custom for widowed women without children (Leviticus 22:13; Ruth 1:12-13). However, Reuben was lying; he had no intention of allowing Shelah to marry Tamar.

Genesis 38:9 - Levirate Matrimony & Onanism

And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother.

The seed should not be his because the first born son would carry on the name of Er so that his dead brother's name would not be forgotten in Israel (Deuteronomy 25:6). Spilled it on the ground is considered a form of contraception from which we get the term "onanism." Although Onan did not masturbate, masturbation ends with the same result - a wasted ejaculation.