Military support or reinforcement, especially from artillery or another military unit helping defend a position.
From French 'appui,' meaning 'support' or 'prop,' derived from Latin 'appodium.' The word entered English military vocabulary during the 18th century through French military terminology.
This is a beautiful example of how military strategy shaped language—'appui' captures the idea that a soldier isn't just fighting alone but has backup firepower, which completely changed how warfare was described in English.
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