1. To take care; be careful; be on guard. — Usually used as a command or warning.
"Look out!" John called, as the car came toward me.
"Look out for the train," the sign at the railroad crossing warns.
2. To be alert or watchful; keep watching.
A collector of antique cars asked Frank to look out for a 1906 gas head lamp.
Compare: EYE OUT, ON THE WATCH.
To watch or keep (a person or thing) and do what is needed; provide protection and care. — Used with "for".
Lillian looked out for her sister's children one afternoon a week.
Uncle Fred looked out for his brother's orphan son until the boy was through college.
Compare: LOOK AFTER.
Source: A Dictionary of American Idioms