inconvertible
Concise Oxford English Dictionary © 2008 Oxford University Press:
inconvertible/ɪnkənˈvəːtɪb(ə)l/
▶adjective not able to be converted.
– derivatives
inconvertibility noun,
inconvertibly adverb.
inconvertibility noun,
inconvertibly adverb.
'inconvertible' also found in these Oxford entries:

