expedition意思是"调查、考察"。fishing expedition可能来自出海探寻鱼群行踪的远征，鱼儿潜游在茫茫大海里，可真是捉摸不定，要找到他们得靠机遇，意义已经引伸到手段不当的调查；（为诬陷他人或搜集新闻材料而进行的）非法摸底调查、审前盘问、非法调查。

【例句】
The senator says the investigation is a fishing expedition by his enemies to see if they can find anything he has ever done that might hurt his political career.
这段话说：那位参议员声称，这次调查是他政敌的蓄意盘查，目的是搜罗挖掘他是否有不端行为，以便损毁他的政治生涯。

When your car got broken into, those two officers were on a fishing expedition.
当您的车子被窃，这两个警察是在摸底。