Siress is a Colonial title used to denote females of some important stature.
For example, female members of the Quorum of Twelve had the title of Siress, like Siress Tinia.
"Siress" is similar to the use of the title "Dame" as a British honorific for a UK citizen with a lifetime contribution to the United Kingdom. Actresses Dame Judith Anderson and Dame Judi Dench were conferred this title.
The male equivalent of this honorary title in the Original Series is "Sire."