The term 'Be Quietly Furious' can be summed up as being in a state of intense anger while keeping it hidden. This is often referred to as 'SEETHING', which is a 6 letter word that can be used to describe this feeling. SEETHING is an emotion that is characterized by a deep-seated anger that is not expressed outwardly, but rather kept inside. It is a feeling of intense frustration and rage that is kept hidden from others.