E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Sublime Porte (The). The Ottoman empire. It is the French for
Porta Sublma, the lofty gate. Constantinople has twelve gates,
and near one of these gates is a building with a lofty gateway
called Bab-i-humajun. In this building resides the vizier, in
the same are the offices of all the chief ministers of state,
and thence all the imperial edicts are issued. The French phrase
has been adopted, because at one time French was the language of
European diplomacy.