A brief outline or cursory description, as in Let me give you a thumbnail sketch of the situation. This idiom alludes to drawing a picture no larger than a thumbnail. [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.