What is the etymology of "look out"?

What does English idiom "look out" (or "watch out") come from? When you want to warn somebody.

Usually, in case of a danger, it is better to hide rather than move your head out to see.

(I know it is an idiom, but maybe there is some reason in it)

Solution 1:

Look out

Definition To be watchful or careful; take care, be vigilant, be on the lookout or be careful; "Watch out for pickpockets!"

Look out origin 1690–1700; noun use of verb phrase look out

Etymology: Lookout (n) also look-out, "person who stands watch or acts as a scout," 1690s, from look + out.

Verbal phrase look out "be on the watch" attested from c.1600.

It appears that the phrasal verb expression look out originates from the noun lookout (one word) which meant ....

Source Century Dictionary: http://www.global-language.com/CENTURY/