He stood in the corner with his arms crossed over his chest, looking unapproachable.

crossed 是动词 cross 的过去分词形式，在这里用作形容词，表示"交叉的"、"交叠的"。cross 的本义是“穿过、横过”，过去分词 crossed 引申为“交叉的、交叠的”。