Cooked slowly in liquid at low heat, or slang for drunk or annoyed.
From Old French 'estuve' meaning a heated room or bath. Later applied to cooking method where food is heated gently in liquid. By the 1600s it gained slang meanings related to being heated up emotionally or literally drunk.
A stewed person is 'heated up'—just like stewed meat simmering in a pot! This metaphor shows up in many languages where being drunk or angry means being 'cooked' or 'heated.' Our bodies even raise temperature when we're angry or embarrassed.
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