His sudden announcement about leaving completely blindsided everyone.

blindsided 是动词 blindside 的过去式形式。blindside 由 blind（盲目的）和 side（侧面）组成，字面意思是"从盲区攻击"，引申为"出其不意地打击"或"使措手不及"。在句中表示"让人完全没预料到"的意思。