1. not likely to be true or to occur or to have occurred

- an improbable event

Similar word(s): supposed, unlikely

2. having a probability too low to inspire belief

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

3. too improbable to admit of belief

Similar word(s): incredible, unbelievable, marvellous, marvelous, tall

Sentences with improbable as an adjective:

- It's highly improbable that aliens abducted you.

- Due to the loss of power, it is improbable that we will begin on time.