Jeremiah 31:19

Surely after that I was turned, I repented After I was enlightened and impressed with a due sense of my duty, and of the many deviations from it of which I had been guilty, and after my will was subjected to the will of God, I straightway became a true penitent, and expressed my repentance by all the outward and inward signs of an unfeigned sorrow for, and hatred to, my past conduct. And after that I was instructed Respecting my sin and folly, in forsaking the fountain of living waters for the broken cisterns that could hold no water; I smote upon my thigh Through the most poignant grief and concern, and with indignation against myself. I was ashamed, yea, even confounded At my own stupidity and frowardness, and could neither with any confidence look up to God, nor with any comfort reflect upon myself; because I did bear the reproach of my youth The burden of my former sins lay heavy upon my mind, and I became sensible that all the calamities and reproaches I had undergone were the due deserts of my offences. Ephraim smote upon his thigh, as the publican upon his breast. We find, both in the Holy Scriptures, and other ancient records, smiting on the thigh mentioned as an expression of great surprise and concern. Thus God commands Ezekiel to cry and howl, on account of the sword that should be upon his people, and to smite upon his thigh. We find the same custom noticed by Homer, Iliad 16. 50:124.

αυταρ Αχιλλευς Μηρω πληξαμενος Πατροληα πρωσεειπεν,

Divine Achilles view'd the rising flames, And smote his thigh, and thus aloud exclaims, Arm, arm, Patroclus! POPE.

It is also noticed by Xenophon, Cicero, and others. The meaning is, when Ephraim was made sensible of his sinfulness he showed marks of real contrition.