The term 'Frankfort Refusal' is a reference to the refusal of the Weimar Republic to sign the Treaty of Versailles in 1919. The treaty was signed by the Allied Powers, but the Weimar Republic refused to sign it, and the term 'Frankfort Refusal' was coined to refer to this refusal. The crossword solution for this term is 'NEIN', which is the German word for 'NO'. This is a fitting answer, as it reflects the Weimar Republic's refusal to sign the Treaty of Versailles.