He said it off the top of his cuff

This is one from “the master”, and it shows.  Subtle and brilliant, it is a  mash up of two similar meaning idioms – “off the cuff” (speaking spontaneously without rehearsal) and “off the top of his head” (saying something without thinking about it first).   Combining these two idioms into one shows “the master’s” continued economical use of the English language.