他關心你，照顧你，愿意傾聽你的心事，也愿意把肩膀借給你依靠，可是，他不是你的男朋友。你們只是好朋友，他是你的“男閨蜜”。

Guy best friend is your non-romantic male soul mate. Usually when a girl is deprived of a brother, this male ends up being the replacement. They are as close as siblings and he means the world to her. He is referred to as the "guy best friend" usually when the girl has best friends who are mostly girls.
Guy best friend（男閨蜜）就是跟你沒有戀愛關系的男性靈魂伴侶。如果一個女孩失去了兄長，這位男士就會成為替代品。他們親如兄妹，他是她的全部。因為這個女孩的好朋友大多都是女性，所以，這位男性好友就被稱為“男閨蜜”。

While most of the time these two remain best friends, it is usually the male that tends to fall for the female. Females who do fall for their guy best friend usually keep it quiet in fear of losing their best friend.
大多數情況下，兩人都會一直保持好朋友關系，不過，男方通常會對女方產生情愫。如果女方發現自己對男閨蜜產生感情，一般不會表白，因為怕會失去這個好朋友。

For example:

Rose says that Victor is just her guy best friend. She has no feelings for him whatsoever.
羅斯說維克多只是她的男閨蜜，她對他沒感覺。

（中國日報網英語點津 Helen）