Definition of Mind-boggling

Babylon English Dictionary
amazing, overwhelming
something said that is impossible to imagine, something said that is very difficult to comprehend
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Mind-boggling Definition from Language, Idioms & Slang Dictionaries & Glossaries
ENGLISH IDIOMS 2.EDITION
too many to count, more than you can imagine The potential for profit is mind boggling - too large to believe.
English Idioms WM 1.3a
too many to count, more than you can imagine
The potential for profit is mind boggling - too large to believe.
WordNet 2.0

Adjective
1. intellectually or emotionally overwhelming; "a mind-boggling display"; "a mind-boggling puzzle"
(similar) impressive