The term 'REALLY THIN' can be described by three crossword solutions: BONY (4 letters), LANK (4 letters), and ALLSKINANDBONES (15 letters). BONY is an adjective that means thin and often BONY in appearance. LANK is an adjective that means tall and thin. ALLSKINANDBONES is an idiom that means very thin, as if all the flesh has been removed from the body. All three words describe someone or something that is very thin.