means A dish of meat and vegetables cooked slowly in liquid over low heat — and, by extension, the agitated worry of someone left to simmer in their own thoughts.

from From Old French 'estuve,' a heated room or bath, tied to a Vulgar Latin word meaning to vaporize with steam — itself likely drawn from Greek 'typhos,' smoke or steam. The English 'stew' first meant a steam bath or hot room, then a public bathhouse (which, in medieval cities, often doubled as a brothel — hence 'the stews' for a red-light district). Only later did the cooking sense win out, the slow simmer keeping the original heat-and-steam idea alive, while 'in a stew' for fretful agitation carries the bubbling metaphor into the mind.