胸元 (munamoto: breast; pit of stomach) chest + origin
僕のどこを叩いてもいいが、胸元だけは避けてくれ。
Boku no doko o tataite mo ii ga, munamoto dake wa sakete kure.
You can hit me anywhere you like, but avoid the chest area.僕 (boku: I (for men))
叩 (tata(ku): to hit)
避 (sa(keru): to avoid)