The term 'unbelievable person' can be described by the crossword solution 'LIAR, ' which is a four-letter word. A LIAR is someone who deliberately tells untruths or falsehoods, often to gain an advantage or to avoid repercussions. Liars may also be motivated by a desire to protect themselves or others from harm. Lying is considered to be a form of deception and is generally frowned upon in most societies.