What does "inconceivably" mean?

In an inconceivable manner, not believable or understandable.

Inconceivably

Common synonyms include: astonishingly, exceptionally, extraordinarily, immeasurably, impossibly, incredibly.

adv

adv ·Rare ·Advanced level

Adverb

Adverb

"The Sahara Desert is inconceivably vast."

From inconceivable + -ly.