The story
* (US: best, or high quality): Possibly from the higher quality, thus more expensive, thus less often used liquors being stored on the less accessible top shelf behind a wet bar. * (British: material with adult content): Such material is often stored on the top shelf in a newsagent's shop, where it is both less visible and harder to reach without being seen.
First known use, by meaning
- from late 19th c. Best, or high quality.
Etymology from Wiktionary, CC BY-SA 4.0.