While searching for words to substitute "godless" in Thesaurus, I stumbled across that word. After searching in google for a while, I could not find any clear definition other than synonyms to this word, nor can I find the origins of this word. So, does it just mean no god? I reckon searching for synonyms often comes up with words close, but not quite similar to the exact definition of my query.
Diamorphic is defined as "Existing or occurring in two distinct forms; exhibiting dimorphism." by the OED, its etymology being: Greek δίμορϕος of two forms ( < δι-, δίς twice + μορϕή form) + -ic suffix.
So I would hazard a wild guess at adiamorphic meaning "not of two forms", and further suggest that it implies being of one form only.