The word dernier is not an English word. It is a word in French. The word dernier means last in French. It is the masculine form. In case of a feminine noun, the word dernier is spelt with an e at the end, as in derniere.
The word dernier (the masculine form of the word, that is, one that does not end with the letter e) is used in such French phrases (their literal English translations are given in brackets) as jugement dernier (The Last Judgement, a fresco by Michelangelo, located in the Sistine Chapel in the Vatican), dernier theoreme de Fermat (The Last Theorem of Fermat, a mathematical theorem)
The word dernier is also used in movies like "le dernier tango a Paris" (The Last Tango at Paris, a French-Italian film by director Bernardo Bertolucci), "le dernier combat" (The Last Combat, a film), "le dernier empereur" (The Last Emperor, a film by Bertolucci) and dernier banco (a film) and le dernier train (the last train).
The word dernier (the masculine form of the word, that is, one that does not end with the letter e) is used in such French phrases (their literal English translations are given in brackets) as jugement dernier (The Last Judgement, a fresco by Michelangelo, located in the Sistine Chapel in the Vatican), dernier theoreme de Fermat (The Last Theorem of Fermat, a mathematical theorem)
The word dernier is also used in movies like "le dernier tango a Paris" (The Last Tango at Paris, a French-Italian film by director Bernardo Bertolucci), "le dernier combat" (The Last Combat, a film), "le dernier empereur" (The Last Emperor, a film by Bertolucci) and dernier banco (a film) and le dernier train (the last train).