The term was arguably coined by RMS and his full statement was:

> “Free software” means software that respects users' freedom and community. Roughly, it means that the users have the freedom to run, copy, distribute, study, change and improve the software. Thus, “free software” is a matter of liberty, not price. To understand the concept, you should think of “free” as in “free speech,” not as in “free beer.”

https://www.gnu.org/philosophy/free-sw.html

Sometimes beer happens to be free, in which case it is referred to as "free beer". It's just an example.