The phrase "boggle the mind" is used to describe something that is astonishing or perplexing. There are several synonyms for this phrase that can be used to express similar concepts. One such synonym is "astound", which means to shock or surprise someone greatly. Another option is "bewilder", which means to cause confusion or puzzlement. "Flabbergast" is another synonym that expresses a sense of shock and disbelief. "Stagger" is another word that can be used to describe something that is overwhelming or astonishing. All of these synonyms can be used interchangeably with "boggle the mind" to convey a sense of wonder and surprise.