köken: Derived from syster by a colloquial formation changing certain /s/ consonant clusters into -rr-. Compare Gustav > Gurra and smaskig > smarrig (“yummy, tasty”).
Etymology tree Esperanto fratino Esperanto -njo Esperanto franjo From fratino (“sister”) + -njo (affectionate female diminutive, applied to the first or first two syllables of the word).