I struck my THIGH; I was ashamed

[tonychanyt]

ESV, Jeremiah 31:

19 For after I had turned away, I relented, and after I was instructed, I struck my thigh [H3409]; I was ashamed, and I was confounded, because I bore the disgrace of my youth.’

Ellicott explained:

The “smiting upon the thigh” is, like the Publican’s “smiting on his breast” (Luke 18:13), an Eastern expression of extreme grief.

NIV:

After I strayed, I repented; after I came to understand, I beat my breast. I was ashamed and humiliated because I bore the disgrace of my youth.'

H3409 literally meant "thigh". NIV translated it as "breast" here to convey the same idea of grief.