Nancy was English slang for 'buttocks' in the 1800s, but by the turn of the century the description changed to "face of an effeminate (Nancy) or gay man". Variations include nancy boy and nance. So the phrase is used when someone looks like a spanked bum, all red and embarassed.
"Whats wrong? Your face looks like a Slap Nancy!"
by Thenamesbanana September 03, 2009