He publicly renounced his noble title.

renounced 是动词 renounce 的过去式。renounce 由前缀 re-（表示"反向、反对"）和词根 nounce（来自拉丁语 nuntiare，意为"宣布"）组成，字面意思是"反向宣布"，引申为"正式声明放弃、拒绝承认"的意思。