Keep an lookout is one of several idioms that have to do with having ‘alert’ eyes and being watchful or noticing something or someone. 1
Also:
be on the lookout
keep a sharp lookout
Meaning of Idiom ‘Keep a Lookout’
To keep a lookout for someone or something means to be watchful or alert, especially to the possibility of danger, suspicious activity, or being discovered.
Similar idioms:
keep an eye out
keep your eyes peeled
keep your eyes open
Examples of Using ‘Keep a Lookout’
“Let’s get all this old junk out of the shed. Keep a lookout for spiders.”
“The water is shallow in this part of the river. Sit up front in the bow and keep a lookout for rocks.”
“You stay in the car and keep a lookout for the cops.”
“You have to keep a lookout for rattlesnakes around here!”
“Mr. Miller, ride back for the coffin cart.” “Yes, sir.” “The rest of you, keep a sharp lookout.” — Sleepy Hollow (1999)
“In these cases, all police divisions are asked to keep a lookout for anyone spending large sums of money.”— Dial M for Murder (1954)
“You should probably get some sleep. I’m gonna stay awake and keep a lookout.” — Monsters (2010)
“Keep a lookout. Honk the horn if you see anything suspicious.” — We Have a Ghost (2023)
Origin
Used since the mid-1700s.
The term lookout, since the 1690s, has referred to a person who stands watch, especially on a boat, or acts as an advanced scout. It derives from the verb phrase look out, meaning ‘be on the watch for.’