1. beyond belief or understanding

Similar word(s): flimsy, unconvincing, astounding, dumbfounding, dumfounding, fabulous, improbable, marvellous, marvelous, tall, undreamed, undreamt, unimagined, incredulous, implausible, unthinkable, incredible

2. having a probability too low to inspire belief

Similar word(s): implausible, improbable, unconvincing, unlikely

Sentences with unbelievable as an adjective:

- The most unbelievable thing happened to me today!

- This restaurant makes unbelievable pizza!

- His excuse seems rather unbelievable.