Rubin's Sign Share definition Flag The brown stain left on underwear from improper wiping after a bowel movement. She knew the one night stand would not be repeated when she woke up in the morning and saw Rubin's Sign in his boxers. Rubin's Sign by Flatbrookdad February 1, 2017 0 0 Get merch Get the Rubin's Sign mug. Buy Now — $32.95 See more merch