literally, "bringing sexy back". i.e Justin Timberlake's song "Sexy back". Can also be used with a sarcastic connotation
"Those man-boobs are SO sexy back"
"OMG GIRL THOSE NEW PIX ON YO MYSPACE ARE SEXY BACKKKK"
girl 1: omgg look at that old guy in the red speedo
girl 2: ughhh SEXYBACK