The term 'SAD ENDING' can be broken down into three separate words: ISM, DEE, and NESS. ISM is a suffix that is used to form nouns that refer to a particular belief, doctrine, or practice. DEE is an informal term for a female friend or companion. NESS is a suffix that is used to form nouns that refer to a quality or state of being. Together, these three words can be interpreted as referring to a situation in which a female friend has a particular belief or practice that leads to a SAD ENDING.