1
general::
Also, watch out. Be careful, be watchful, as in Look out that you don't slip and fall on the ice, or Watch out! There's a car coming. [c. 1600] Also see LOOK OUT FOR.
American Heritage Idioms
2
general::
Phrase(s): look out
Go to watch out for someone or something., Phrase(s): look out (of) something
to gaze outward from inside something. (Of is usually retained before pronouns.) • Look out of the window and see if it is raining. • I looked out of the door to see what the weather was like.
McGrawhill's American Idioms And Phrasal Verbs