watch out

Concise Oxford English Dictionary © 2008 Oxford University Press:
watch/wɒtʃ/

▶verb

• 1 look at attentively.

  ■ keep under careful or protective observation.

  ■ (watch for) look out for.

• 2 exercise care, caution, or restraint about.

  ■ (watch out) be careful.

• 3 maintain an interest in.

• 4 archaic remain awake for the purpose of religious observance.

▶noun

• 1 a small timepiece worn typically on a strap on one's wrist.

• 2 an act or instance of watching.

  ■ a period of vigil, typically during the night.

• 3 a fixed period of duty on a ship, usually lasting four hours.

  ■ (also starboard or port watch) the officers and crew on duty during one such period.

  ■ a shift worked by firefighters or police officers.

• 4 (also night watch) historical a watchman or group of watchmen who patrolled and guarded the streets of a town at night.

– phrases
be on the watch be on the alert for danger or trouble.
keep watch stay on the lookout for danger or trouble.
watch one's (or someone's) back protect oneself (or someone else) against danger from an unexpected quarter.
the watches of the night literary waking hours during the night.

– derivatives
watcher noun.

– origin OE wæcce ‘watchfulness’, wæccende ‘remaining awake’; rel. to wake1.