The surname Kęsgaila comes from the first name of Mykolas Kęsgaila son of Valimantas, whose elder brother Jaunutis son of Valimantas was granted the Zadora coat of arms in the Union of Horodło of 1413. In line with the common practice for nobles' names after the Christianization of Lithuania, after his baptism, the Christian name *Mīkālas (Michael) was added to a pagan Lithuanian name, which can be reconstructed as *Kensgaĭla (from kęsti ("be patient") and gailas ("strong")). His son Jonas Kęsgailaitis, mentioned in written sources in Latin as Johannes Kyensgalowicz, was a bearer of the name as a patronymic, which subsequently evolved into a family name. Its Polonized form Kieżgajło is most common in Polish historiography over other variants, such as Kezigal, Kieżgajłła, Gezgajło, Kiezgajło, Kieżgajłło, Kierżgajłło, Kieżgałło and Kieżgało. Kęsgaila (pluralKęsgailos) is a modern Lithuanian spelling of the original family name.