Bouché à l'émeri

Literally: Stoppered with emery

Example: “Je lui ai expliqué trois fois, mais il est bouché à l'émeri.” — I explained it to him three times, but he's as thick as a brick.

English equivalent: Thick as a brick

Origin: Glass flasks were once sealed airtight by grinding both the stopper's outside and the neck's inside with emery, an abrasive, until the fit was perfect. The slang bouché, 'plugged', for stupid dates from the 18th century; the émeri variant appeared in the early 20th. The image is exact: nothing escapes such a vessel, but nothing gets in either.