What does Themistō mean in Latin?

Themistō, a proper noun, means “Themisto”.

What is the genitive of Themistō?

The genitive singular is Themistūs. Themistō is feminine, fourth declension.

In classical Latin, [tʰɛˈmɪs.toː]; in ecclesiastical (church) Latin, [teˈmis.to].

proper noun · femininefourth declension

Themistō f sg (genitive Themistūs)

Classical[tʰɛˈmɪs.toː]Ecclesiastical[teˈmis.to]

Themisto
Athamas Aeoli filius habuit ex Nebula uxore filium Phrixum et filiam Hellen, et ex Themisto Hypsei filia filios duos, Sphincium et Orchomenum, et ex Ino Cadmi filia filios duos, Learchum et Melicerten. Themisto, quod se Ino coniugio privasset, filios eius interficere voluit. [...] Themisto cognita re ipsa se interfecit., Hyginus, fabulae, Themisto; in: Hygini fabulae, edidit Mauricius Schmidt, Jena, 1872, page 38

From Ancient Greek Θεμιστώ (Themistṓ).