From *küčtüg, a regional (East Turkic) version of *küčlüg "strong", from Proto-Turkic*gǖč(“force”) + +lüg "affix of possession or quality".
Compare cognates:
Kazakhкүшті(küşti),
Kyrgyzкүчтүү(küçtüü),
Southern Altaiкӱчтӱ(küčtü),
Khakasкӱстiг(küstig),
Tuvanкүштүг(küštüg),
Yakutкүүстээх(küüsteex, “strong”).
See more cognates at көслө(köslö).