1 general:: Phrase(s): a thumbnail sketch a brief or small picture or description. • The manager gave a thumbnail sketch of her plans. • The student wrote a thumbnail sketch of his project.
McGrawhill's American Idioms And Phrasal Verbs
2 general:: 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]
American Heritage Idioms