Neapolitan and other southern dialects,
It defines a state of melancholic nostalgia towards something that is currently missing in your life and won't be likely recovered. Usually, it accompanies people relocating to a new place far away from home, but it can also be extended to more general reflections about life and our existential condition.
"Ajere m'agge curcato, penzanne a la vita mije ultimamente e quille che ce vole pe te fa l'abbetudene de vive all'estere: tanne, l'appocundria m'è scuppiate mbiette!"
"Yesterday I was lying in bed thinking about my latest life events and what it takes to adapt to a new life abroad: that's when appocundria started to kick in!"