Sonder Share definition Flag n. Pseudophilosophical guff memetically spread by the tumblr blog "The Dictionary of Obscure Sorrows". No academic grounding whatsoever, can be figured out through common sense. I just had a sonder.A what? Stop talking shit! Sonder by Ma1nFr4me May 28, 2013 0 0 Get merch Get the Sonder mug. Buy Now — $32.95 See more merch