Blow me!

This is a mild expression of surprise or perplexity rather than an injunction to perform fellatio. It dates from the early 19th century and is probably a derivation from the earlier naval expression “blow me down”, as in a gale, which dates from the 18th century. It has various forms: “blow me down with feather!” “Blow me over!” etc. To blow also means to brag or boast; this meaning is much older and dates from The Middle Ages. See also Blowed and Blowhard.