He had pleaded with his superior that he wasn't familiar enough with that part of the country, all in vain.

pleaded 是动词 plead 的过去分词形式。plead 的意思是"恳求、请求"，通常用于正式场合或法律语境中，表示强烈而正式的请求。pleaded 在这里表示"曾经恳求过"的动作。