'caveat emptor'
I presume that's where the current use of caveat as a noun stems from. This is a common side effect of popular set phrases where some or all of the constituent parts are uncommon and/or foreign words. Something gets lost in translation.
Not so sure. I did an ngram of caveat and caveat emptor, and caveat on its own appears to be much more frequent. Of course the caveat dataset will contain the caveat emptor data as well, but the simple caveat graph isn't in step with it.
That ngram suggests caveat became
really popular as a standalone word in the 1960s, and has continued to grow in popularity since then. I imagine that's when it became detached from its original source and meaning to take on a life of its own.
As to what that original source was, I can't claim to know, but I still feel it's most likely to have been abstracted from a set phrase, such as caveat emptor. Or it could be a bit of legal jargon that has made the transition into mainstream language. Legalese is often the original source for this kind of thing.
Anyway, it's clear that
something happened in the 1960s to help it gain a foothold in everyday use. I bet it's all that swinging that went on then. Bloody hippies. No respect for standards.