See to the welfare of, as in Mary was assigned to look out for the youngsters on the playground. Similar to look after, this expression appears in such terms as look out for number one, meaning “see to one's own best interests,” as in Looking out for number one is Barbara's first priority. Versions of this expression, such as take care of number one, date from 1700.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.