use one's head
Think, have common sense, as in Use your head, Martin. No one's coming out in this weather. This idiom uses head in the sense of “brain” or “intellect,” a usage dating from the late 1300s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.