Charles Ellicott Commentary Jeremiah 31:19

"Surely after that I was turned, I repented; and after that I was instructed, I smote upon my thigh: I was ashamed, yea, even confounded, because I did bear the reproach of my youth." — Jeremiah 31:19 (ASV)

After that I was turned. —The words have been referred by some commentators (Hitzig) to the previous turning away from God—the apostasy of Ephraim; but the repetition of the word that had been used in the previous verse makes it far more natural to connect it with the first movement of repentance. The smiting upon the thigh is, like the Publican’s smiting on his breast (Luke 18:13), an Eastern expression of extreme grief. So in Ezekiel 21:17 we have the smiting of the hands together as a symbol of anger, which is also sorrow. In Homer (Odyss. xiii. 193) we have the very gesture here depicted—

“And then he groaned, and smote on both his thighs
With headlong hands, and so in sorrow spoke.”

The reproach of my youth — i.e., the shame which the sins of his youth had brought upon him.