"The origin of the word 'nosferatu' is obscure. The first recorded reference in print was in a magazine article in 1885, and three years later in a travelogue entitled The Land Beyond the Forest, both written by the British author Emily Gerard. The travelogue described the country the country of Transylvania (its latin name translates as 'the land beyond the forest'). In both she stated that "nosferatu" as the Romanian word for 'vampire,' but there is no known identifiable corresponding word in any form of the Romanian language, ancient or modern. The closest are necuratul ('the devil') and nesuferitul ('insufferable one').
An alternative explanation, which has been accepted by many writers, is that 'nosferatu' is derived from an old Slavonic word nesufur-atu, which was apparently itself derived from the Greek nosophoros, meaning 'plague-carrier' or 'disease-bearing.' The obvious objection to the etymology is that Romanian and other Slavonic languages are Romance in origin and contain very few words from the Greek. It's also significant that, though the word nosophoros is a valid compound word in the Greek language--meaning that the two parts of the compound word are individually valid and are correctly combined--there's no evidence that the word ever existed in any phase of the Greek language. So this suggested etymology relies on an unknown Greek word that somehow gave rise to an unknown Romanian words, which seems fairly unlikely.
It has also been suggested that nesufur-atu/nosferatu was a technical term in Old Slavonic that had migrated into common usage, but never appeared in a Romanian dictionary. That is a somewhat difficult argument to sustain, given that the sole purpose of a dictionary is to record words in common usage, and it would be reasonable to expect thtat it would have been recorded somewhere.
So we'll probably never know exactly where 'nosferatu' originated, but the balance of probability is that Emily Gerard either misheard a Romanian word of was misinformed.
Bram Stoker, of course, used the word in his novel Dracula, but his usage suggests that he probably believed it meant 'not dead' or 'undead' in Romanian, not "vampire," and he used it as a calque or loaned word."
----from endnotes to James Becker's novel, The Nosferatu Scroll.