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.
It’s OK To Use “They” To Describe One Person: Here’s WhyHas someone ever asked you to refer to them as they instead of him or her? Are you hedging because you can't possibly refer to one single person as they? What if we told you that they has been used to refer to just one person since at least the 1300s?
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.