A person who signals with flags or hand signals, or someone who marks locations with flags.
From 'flag' (Old Norse flag meaning to flap) combined with the agent suffix '-er' (one who does). Commonly used for traffic control workers in the 19th-20th centuries.
Road construction flaggers are actually performing a safety function that dates back to railroad days when they'd signal approaching trains with flags and lanterns—it's one of the oldest forms of communication technology!
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