What is "vyachko"

Vyachko of Koknese, also Vetseke of Kokenhusen (, , ) was the ruler of the Principality of Koknese in present-day Latvia, a vassal of Polotsk, who unsuccessfully tried to establish himself as a local ruler first in Latvia and then in Estonia, and fought against the expansionism of the Livonian Knights at the turn of the 13th century.