I know it's first uttered in Cameron's Aliens (1986). Once. It's a word that seems was coined for the movie itself, but one constructed of well-known root words that seem to mean "alien-shaped".
In other words, it's in the same category as "humanoid" and "quadruped". In fact, something that is neither of those might well be "xenomorph" by default.
Can anyone offer any explanation as to why (and when, and how) this word was chosen by the fan community to represent the aforementioned nasties in this franchise?
It would seem to me that if you just had to give them a name, naming them after the artist that created the initial artwork (H. R. Giger) would be the most appropriate thing to do.