Walking or traveling around an area regularly to watch for trouble or keep it safe.
From French 'patrouille,' which came from 'patrouiller' meaning to wade through mud. It originally referred to soldiers walking through mud on night watch, which evolved into the modern meaning of routine surveillance.
Police officers originally walked the same routes on foot every day so people would know they were around—this is why the word 'patrol' still often means walking, even though patrol cars now do most of it!
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