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 WordNet
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