What’s the rarest word?

Have you ever heard of the term quincunx?

Among the rarer words, we find deliquescent, which is an adjective referring to a process of turning into liquid or having a tendency toward liquefaction. Then there's flabbergast, a verb that means to astonish someone to a significant degree. Other examples include flimflam, floccinaucinihilipilification, limerence, loquacious, obdurate, and omnishambles.