Jeremiah 31:19 breast or thigh?
The original Hebrew word used is yarek (Strong's 3409) which is usually translated as "thigh, loin, side, base". This is one of many examples of showing grief or shame.
Why does the NIV use "breast" instead of thigh?
The beating of the breast is another way of expressing grief or shame. The second paragraph of the Insight on the Scriptures article "Attitudes and Gestures" subheading Grief, Shame describes different ways the Orientals express their feelings:
Cutting off or pulling out hair; beating breast. Cutting off the hair (Job 1:20), pulling some of the hair out of one’s own beard (Ezr 9:3), covering the head (2Sa 15:30; Es 6:12), covering the mustache (Eze 24:17; Mic 3:7), and laying one’s hands on his own head denoted grief or shame, even to the point of being stunned. (2Sa 13:19; Jer 2:37) Some believe that the latter gesture signified that the heavy hand of God’s affliction was resting on the mourner. Isaiah walked about naked and barefoot as a sign of the shame to come upon Egypt and Ethiopia. (Isa 20:2-5) Under the feeling of unusual grief or contrition one might beat the breast in grief (Mt 11:17; Lu 23:27), or slap the thigh for regret, shame, and humiliation or mourning.—Jer 31:19; Eze 21:12. (bold mine)
So a direct translation of Jeremiah 31:19 would use the word "thigh" but may be unfamiliar to some readers. While the use of the word "breast" still conveys the same idea and may be understood by more people.