On my system (Darwin 15.5.0),
man(1) opens as follows:
NAME man - format and display the on-line manual pages
The file the page is formatted from, however, is clearly on disk:
% man -w man /usr/share/man/man1/man.1 % file `man -w man` /usr/share/man/man1/man.1: troff or preprocessor input text
So, "on-line" in this case does not mean "online," as in, "somewhere else accessible over the Internet."
Does "on-line" just mean that my system is powered on? If so, why bother specifying that in the first place, i.e., isn't it obvious that I'm reading a page that the formatter processed? Or, when the description was written, was it a huge deal to have a manual on disk because most "manuals" then were paper volumes? Is this usage of "on-line," hyphen and all, still common in computing?