Just to back that up, it had a clear meaning because -less is a valid suffix to append to English words. When grandma runs out of cookies she is cookieless (when a website doesn't use cookies it is also cookieless). It doesn't have to be "in the dictionary" to make sense in conversation.
A quick search of usenet shows "serverless" being used in 1994. It wasn't a term or a buzzword, it wasn't common, it was just English: https://groups.google.com/d/msg/comp.os.linux.misc/r76oNl98C...