To shock or surprise or astonish one; also, to lose one’s mind, to go crazy. This slangy phrase dates from the mid-1960s, when hippie culture and anti-establishment feelings were at their height, and when it also meant to have a drug-induced experience. By 2000 the most common usage involved amazement, as in “He managed to juggle eight oranges at a time—it blew my mind.” See also blow away.