I recently read a forum post that asked about “poor Japanese”. For some reason, this struck me as both bad grammar (using an adjective, ‘Japanese’, as a noun) and offensive. I noticed that similar phrases, substituting words such as ‘gay’ or ‘black’ for ‘Japanese’, also struck me as offensive. But phrases like ‘an American’, ‘a lesbian’, or ‘a Hispanic’ don’t have the same offensive capability.

The common thread seems to be that when X can function both as an adjective and as a noun, violating grammar by referring to someone as ‘a/an X’ is offensive, whereas it isn’t if X can also function as a noun. I’m not sure why a simple grammatical error can sound so grating to the ear. Perhaps it has to do with the brain filling it in as ‘a gay man’, for example, and by omitting the word ‘man’ the speaker is subtly denying the subject’s humanity. Or maybe, by using an adjective as a noun, the speaker implies that the subject is solely identifiable by that adjective and conforms to all the associated stereotypes.

Regardless of the explanation, the phenomenon is likely rooted in the listener taking offense against prejudice and stereotype, as when a non-stereotyped adjective, such as ‘tall’ or ‘brown-eyed’, is substituted for X, the phrase ‘a/an X’ ceases to be offensive, merely ungrammatical.