Genesis 36:8-16: And Timna was concubine to Eliphaz Esau's son; and she bore to Eliphaz Amalek.
Who was Timna?
Sanhedrin 99b: Timna was a royal princess, as it is written, alluf [duke] Lotan, alluf [duke] Timna... Desiring to become a proselyte, she went to Abraham, Isaac and Yaakov, but they did not accept her. So she went and became a concubine to Eliphaz the son of Esau, saying, ‘I had rather be a servant to this people than a mistress of another nation.’ From her Amalek was descended who afflicted Israel. Why so? — Because they should not have repulsed her.
When someone is distanced from the Jewish people and made to feel unfit, the consequences can be disastrous. Timna was turned away three times. The result was Amalek. Remember that the next time you're about to sneer at someone who doesn't wear a hat, or similar.