Today I was listening to a beautiful rendition of 'Let the bright Seraphim' by Lesley Garrett and I thought seraphim would be a plural noun, based on the rules of the Hebrew language. But was the singular seraph?
Yes, my computer's dictionary said it was.
But it's a back-formation, a word formed by deleting the suffix from 'seraphim'. The Princeton WordNet explains a back-formation as 'a word invented (usually unwittingly by subtracting an affix) on the assumption that a familiar word derives from it.' For instance, the verb burgle came into use because of the mistaken assumption that burglar is a noun formed by adding the ar ending.
As William Safire said in the New York Times, "People do not consciously work out the backward step, but they have a sense that burgle is to burglar what sail is to sailor."
The Online Etymology Dictionary dates the first use of seraph to 1667, by Milton, and suggests he formed this singular noun by analogy with cherub/cherubim.
I guess Milton wouldn't have had much experience with Hebrew plurals used in English, as they're not common. Nowadays there are only a handful in use. Laurie Bauer, in the book Morphological Productivity, says there are probably only cherubim, seraphim, kibbutzim and goyim in regular use.
After considering all this information I was left with the question: what did people before 1667 call one of the seraphim?
Maybe they just didn't talk about them.