Almost impossible to comprehend

I believe the answer is:

mind-boggling

'almost impossible to comprehend' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for mind-boggling that I've seen before include "Beyond comprehension" , "Astonishing" , "Overwhelming" , "Incredible" , "Almost impossible to comprehend" .)

I've seen this clue in The Guardian.

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