means A person hired to guard a building, property, or area, especially at night, by staying alert and raising the alarm at any sign of trouble.

from A plain compound of "watch" and "man." The "watch" here comes from Old English "wæcce," meaning a state of wakefulness or vigil — the same root that gives us "wake" and "awake." To keep watch was literally to keep yourself awake while others slept. Medieval towns ran on this: the night watch patrolled the dark streets, called out the hours, and shouted when fire or thieves arrived. "Watchman" simply names the human doing the staying-awake.