"Humanism" already means a different thing. If it’s the "fem" prefix that is really hanging you up, just think of all the words using prefixes from other languages that have nothing to do with their meanings. Ambul-, for example, means "to walk," but our ambulances certainly aren’t ambulatory. Frater means "brother," but you can fraternize with men and women whom you aren’t related to. Feminism has that "fem" prefix, but it is concerned with more than just women and is practiced by more than women. The fact that "human" is in the word "humanism" doesn’t mean it’s the non-gendered form of feminism, because English doesn’t work that way.
From a Jezebel comment in response to someone arguing that “humanism” is a better word for the movement than “feminism.”