People who are watching carefully to see if something is coming or happening, or high places from which you can see far into the distance.
From 'look' (Old English 'locian') plus 'out' (Old English 'ut'). The word developed from the 17th century meaning 'a person watching' to also mean 'a place from which to watch.'
On pirate ships, the lookout was the person with the best eyesight posted at the highest point—they'd call down 'Land ho!' or warnings about other ships, making them one of the most important crew members even though they worked alone.
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