A size of paper between royal and crown. Its size is 22 1/2 in. x 17 1/2 in. It is from the French word demi (half), and means demi-royal (a small royal), royal being 25 in. x 20 in. The old watermark is a fleur-de-lis.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894