Although dictionaries list obsolete words such as recomfort and reechy, these words are not appropriate for current use. Invented words or expressions (called neologisms) are too recently created to be part of Standard English. Many fade out of use without becoming standard. YOLO and MOOC are neologisms that may not last. Prequel and e-mail are no longer neologisms; they have become Standard English. Avoid using invented words in formal writing unless they are given in the dictionary as standard or unless no other word expresses your meaning.