Adam Clarke on Jeremiah 31:19

After that I was turned - Converted from my sin, folly, and idolatry.

I repented - To conviction of sin, I now added contrition for sin. Conviction, in this sense of the word, must precede contrition or repentance. As soon as a man sees himself lost and undone, he is convicted of sin; when convicted, he begins to mourn. Thus contrition follows conviction.

I smote upon my thigh - My sorrow grew deeper and deeper; I smote upon my thigh through the extremity of my distress. This was a usual sign of deep affliction. See Eze 21:12. It was the same among the ancient Greeks. So Homer: - Ως εφατ' αυταρ Αρης θαλερω πεπληγετο μηρω Χερσι καταπρηνεσσ,ολοφυρομενος δε προσηυδα.

IL. lib. 15:113. "She spake: and with expanded arms, his thighs

Smiting, thus sorrowful, the god exclaimed."

Cowper. - αυταρ Αχιλλευς Μηρω πληξαμενος Πατροκληα προσεειπεν.

IL. lib. 16:124. "Achilles saw it, smote his thigh, and said."

Cowper.

I have often seen persons in deep grief act thus.