Pseudopseudohypoparathyroidism is, probably, the longest word in the English language that hasn't been specially made up to be the longest word in the English language.
Pseudopseudo etc is a genetically-transmitted disease.
It is also a Sunday, which is supposed to be a day of rest.
So let's, well, rest, shall we?
(Honestly two pseudos in a row...what were they thinking?)
Sunday Rest: pseudopseudohypoparathyroisism. The only interesting thing about this word is the thyroid bit, which comes from the Greek word thureos, which is a door-shaped shield.