Unimaginably means to a degree so extreme that it's almost impossible to picture or comprehend in your mind. When something happens unimaginably fast, it moves so quickly that your brain can't really grasp the speed. When a mountain is unimaginably tall, it's so high that you struggle to conceive of its actual height.

The universe is unimaginably vast: even knowing that it contains billions of galaxies, each with billions of stars, we can't truly picture that scale. The number of grains of sand on all Earth's beaches is unimaginably large.

You might say a roller coaster was unimaginably scary or that a professional athlete's reflexes are unimaginably quick. Sometimes people use this word for exaggeration, like calling their homework unimaginably boring, but the word carries the most power when describing things that genuinely stretch beyond normal human understanding.