That causes the mind to boggle; that is beyond one's ability to understand or figure out; bewildering; mystifying.(adjective)
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Use "mindboggling" in a sentence
"The record, which includes both the CFL and NFL, and the attention he has received had a tired looking Calvillo using the word "mindboggling" a lot a day after the feat."
"What makes it more mindboggling is that it’s popular enough to hold a convention based around it and a computer game is in the works for it."
"Some comments called the move "mindboggling" while others noted he had gotten his life back – while Gulf residents had not."