drop a line
Correspond, write a short letter or note. For example, I hope you'll drop me a line soon. This idiom uses line in the sense of “a few words in writing,” a usage first recorded in 1647.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.