New-fangled notions. When the calendar was reformed by Pope Gregory XIII. (1582), letters used to be dated stilo novo, which grew in time to be a cant phrase for any innovation.
“And so I leave you to your stilo novo.”
Beaumont and Fletcher.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Stilo Novo from Infoplease: