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Synonyms for Indefinable (same or very similar meaning)

WordNet sense 1 (defying expression or description):
indescribable, ineffable, unspeakable, unutterable, untellable

WordNet sense 2 (defying expression):
inexpressible, unexpressible

WordNet sense 3 (not capable of being precisely or readily described; not easily put into words):
undefinable

WordNet sense 4 (not precisely limited, determined, or distinguished):
undefined, vague


From the ODE community, based on WordNetadd/edit