Definition of "Volapük"

...some authors have claimed that the slang of the criminal was a kind of international language for thieves, a Volapük of crime.

1897 April, A. F. B. Crofton, “The Language of Crime”, in Popular Science Monthly‎, volume 50, page 834

The first practical constructed language was the south-west German Pastor Schleyer's Volapük from 1879; its complicated grammar and irregular vocabulary made learning difficult, however. The most successful has been Esperanto, devised by the Warsaw ophthalmologist Ludwig Zamenhof in 1887, that today can count some one million speakers.

2004, Steven Roger Fischer, A history of language, Reaktion Books, page 180