The former smoker gave a speech renouncing his habit of smoking.

renouncing 是动词 renounce 的现在分词形式。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"公开宣布放弃或拒绝"。在句中表示"公开宣布戒除"的行为。