The small, flat nail on your thumb. It also means a very small version of a picture or a short summary of something.
A simple compound of 'thumb' and 'nail' from Old English roots. The figurative sense of 'very small representation' dates from the mid‑19th century, originally referring to tiny drawings. In computing, it was adopted for small preview images.
Calling a tiny image a 'thumbnail' literally compares it to the size of your thumb’s nail. The same idea explains 'thumbnail sketch'—a drawing so small you could almost fit it on your thumb. Your body once again becomes the measuring tool for the digital world.
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