Through someone, by way of someone, as in I sent the gift in care of your parents. This phrase indicates that something is to be delivered to someone at someone else's address. [Mid-1800s] Also see in charge, def. 3.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.