The etymology is inoffensive, but the term itself is still exclusionary by way of "man" also being a gendered term. We are (and should be) more cognisant of these issues these days.
>>lowboy+(OP)
No, you're wrong. "man" is the obvious abbreviation for "manual" when trying to reduce the number of characters used. Not once as a teenager learning unix shell commands did gender (or cultural stereotypes) enter my mind when using the 'man' command, but I guarantee it does when looking at the 'bro' command.