Gun是枪。Top gun来自美国西部刚开拓的年代。当年枪手你死我活地血战拚杀，就为了争个top gun的称号，意思是头号枪手，后来引申为某领域内的高手；精英。

【例句】
The football star hired several of the top guns among defense lawyers all over America. They managed to convince the jury that he was not guilty.
那位足球明星网罗了美国辩护律师中几名精英。这些人设法使陪审团相信被告无罪。

And one chemical industry spokesperson calls her "a top gun" for the environmental movement.
一家化工企业的发言人称她是环境运动的“勇士”。