Where does “Русчук” come from?

Русчук (Bulgarian) comes from Ottoman Turkish روسجق, from Ottoman Turkish ـجق — Diminutive suffix after back vowels.

Ancestry of “Русчук”, step by step

StepLanguageWordMeaning
1Ottoman Turkishروسجق—
2Ottoman TurkishـجقDiminutive suffix after back vowels
Every word from Ottoman Turkish ـجق →Every word from Ottoman Turkish روسجق →