Maybe this is it?

> first analysed in 1912 ... by Polish physicist Marian Smoluchowski. ... popularised by [Feynman] in a physics lecture at the California Institute of Technology on May 11, 1962,

https://en.wikipedia.org/wiki/Brownian_ratchet

It’s an allusion to the Brownian ratchet thought experiment. AFAIK he demonstrated why it does not work.

Surely he was joking.