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Synonyms for Improbable (same or very similar meaning)

WordNet sense 1 (having a probability too low to inspire belief):
unconvincing, unlikely

WordNet sense 2 (having a quality that provokes disbelief):
implausible

WordNet sense 3 (too improbable to admit of belief):
marvelous, tall, marvellous

WordNet sense 4 (beyond belief or understanding):
incredible, unbelievable


From the ODE community, based on WordNetadd/edit