They had written off him before they even met him.

written 是动词 write 的过去分词形式，write 的基本意思是"写"。 off 在这里用作副词，表示"完全地、彻底地"。 write off 是一个短语动词，原意是"勾销（债务）"，引申为"认为...毫无价值"或"放弃...的希望"。