-kunī - of you both; both of you (genitive and accusative pronominal suffix, 2 du.) (GAG § 42 i**)

Old Akkadian.
2 du. pronominal suffixes are also present in Arabic (see -kumā) and MSA (see Mhr. -ki). Akkadian and Arabic forms are based on the plural masculine form, while in MSA 2 du. pronominal suffixes are formed on the basis of the singular. Non-Semitic Afroasiatic evidence (see Eg. -ṯny) points at the primary nature of Akkadian-Arabic picture (Kogan 2009:69).