The debate turned into a bare-knuckle exchange of insults.

bare-knuckle 是由 bare（裸露的）和 knuckle（指关节）组成的复合形容词。bare-knuckle 字面意思是"赤手空拳的"，引申为"毫不留情的、激烈的"，形容不戴拳击手套的搏斗或比喻言辞激烈的交锋。