Mind-boggling

Mind boggling and its sister expression the mind boggles derive from the 16th century meaning of boggle meaning to alarm or frighten. We get the words bogey and bogeyman from the same root. The meaning today is more one of intellectually challenging or confusing rather than alarming. Although the word boggle dates from the 16th century, expressions like the mind boggles and mind-boggling i.e. the introduction of the word ‘mind’ into the equation, date from the mid-20th century.