There are many terms from classic and modern SF that remain unresearched, and the Historical Dictionary of Science Fiction will be continually updated, especially as additional resources are put online. Boing Boing is syndicating new entries from the HDSF on a regular basis. (Read the series introduction.)
To comics fans, the noun inhuman is most familiar as the name of a race of superhumans in the Marvel universe, introduced in the December 1965 issue of Fantastic Four, and especially to the Inhuman Royal Family, a particular team of these. But the word is used more broadly to refer to any non-human being. Though it has a Lovecraftian feel, H. P. Lovecraft himself never used the noun, even if, like many Weird Tales writers, he was rather fond of the adjective, writing about "inhuman evil" and "inhuman squeals" and "shrieks" and "screams" and "cries." (We do not include the adjective in the HDSF; there's nothing particularly science-fictional about it.) The Oxford English Dictionary has evidence for the noun from the 17th century in the sense 'a brutal person', but it's labelled both "obsolete" and "rare." The first person to use the noun in SF appears to be Isaac Asimov in 1940, referring to aliens; it's now more common in fantasy and horror, referring to non-humans of our world world rather than aliens of other ones.