noun
plural -outs
[count] a person who watches an area and warns others if there is danger
a high place or structure from which you can see a wide area
be someone's lookout
Brit informal - used to say that someone who makes a bad decision or does something foolish is responsible for the result and will have to deal with it alone
keep a lookout for
to watch for (something or someone)
on the lookout for
looking or searching for (something or someone)