He hit his head when he slipped and got slightly concussed.

concussed 是动词 concuss 的过去分词形式。concuss 来自拉丁语 concutere（剧烈摇晃），在医学上指"因头部受撞击导致脑部震荡"。concussed 表示"遭受脑震荡的"状态。