The insult equivalent to suck me sideways used in situations resulting in great surprise, happieness, shock, or anger. Often used when something out of the ordinary happens, or you are told about something out of the ordinary happening to someone else. Can also be taken literaly, if a man tells someone to blow him backwards it means blow me so good I cum propeling myself across the room.
Guy 1: dude, that chick just grew a penis in under a minute
guy 2: well, blow me backwards, that's crazy shit right there!