Their team got defeated in the debate competition, losing all credibility.

defeated 是动词 defeat 的过去分词形式，在句中表示被动。defeat 的意思是"击败、战胜"，defeated 表示"被击败的"。 这里 defeat 由前缀 de-（表示"去除、剥夺"）和词根 feat（来自拉丁语 facere，意为"做、完成"）组成，字面意思是"使无法完成"，引申为"击败"。