Watching, guarding, or keeping vigil; positioned in a watchful or protective state.
From Latin excubans, present participle of excubare (ex- 'out' + cubare 'to lie'), originally meaning to lie outside or keep watch, used in Roman military contexts.
This word comes from Roman times when soldiers would literally 'lie outside' fortifications to keep watch—it's a perfect example of how military language becomes part of literary English.
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