back-o'-lantern Share definition Flag When the sweat on your back, seeps through your shirt and imprints a face, resembling a jack-o'-lantern. Look at the back-o'-lantern on that guy!Dude, get a new shirt, you have a back-o'-lantern! back-o'-lantern by Mark-TWaine September 5, 2012 0 0 Get merch Get the back-o'-lantern mug. Buy Now — $32.95 See more merch