(1) A dark cell (usually underground) where prisoners can be confined
(2) The main tower within the walls of a medieval castle or fortress
WiktionaryText
Etymology
dungeon, dongeo(u)n, dungun, donjon from of origin, from *dung "prison, dungeon", *dungjo "underground cellar" from from . Akin to dung "prison, dungeon", dung "cellar", tung "cellar". More at dung
