Or your smooth belly may suddenly sprout an elbow-like bump.

或者是你光滑的肚子上可能突然之间长出一个拳头样的肿块。

The elbow bump is an informal greeting where two people touch, or tap, elbows.

撞肘是一种非正式的问候方式，两个人通过肘部轻触或者撞击来问候对方。