Real world words
Is this something invented by Martin or did it originate elsewhere?
I hate to break it to the in-universe and sci-fi/fantasy-authors-invented-this folk, but these are all real world language.
- As chepner points out in another answer, "ser" is simply Middle English for the Modern English "sir", as the OED says.
- "maester" is Early Modern English for "master". It can be found in William Tyndale's 1534 translation of the New Testament, alongside words such as "faether", "moether", and "broether". (Brooke Foss Westcott (1872). "§ I. Tyndale." A General View of the History of the English Bible. 2nd Edition. London. Macmillan, pp. 51–52)
- "Joffrey": There's a huge range of ways to spell names rooted in "got" + "fridu". Some of them can be seen at "Gotfrid" (The Kurufin's Castle). Spellings such as "Jofry" and "Jofré" are quite real. "Joffrey" and "Robb" (a simple diminutive of "Robert") are first names that have become surnames over the past seven or so centuries, and one can find many people with these names. There was an aristocratic Joffrey family in Vevey, Switzerland from the 15th to the 18th centuries, for example.
There's a more extensive analysis of these real world archaisms done by an OED employee:
More specifically, was there another fantasy author that used "Ser" as a noble title?
As you have seen, plenty have used these real world archaisms. I add one more to follow on from Nate Wells' answer: James Tiptree Jr. who used gender-neutral "Ser" and "Myr". In Brightness Falls from the Air the appendix says:
Myr, Myrrin: Myr serves for Mr., Mrs., Ms., or Miss and is often prefixed affectionately or jokingly to a person's first name or nickname. Myrrin is the plural, corresponding to "Ladies and Gentlemen".
And a character insists upon being called "Ser" instead of "Myr", just as some people in the real world insist upon being a "Sir" when called "Mister":
"And then you are Myr … ?" Kip addresses the taciturn man.
"Ser Xe Vovoka," the stranger corrects him. […]
Kip recalls that "Ser" is a technical honorific, somewhere beyond "Doctor". Not to call him "Myr".