Where does “косъ” come from?

косъ (Old East Slavic) comes from Proto-Slavic kǫsъ, from Proto-Slavic kǫsati, from Proto-Slavic kǫ̑sъ — piece.

Ancestry of “косъ”, step by step

StepLanguageWordMeaning
1Proto-Slavickǫsъpiece
2Proto-Slavickǫsatito bite
3Proto-Slavickǫ̑sъpiece
Every word from Proto-Slavic kǫ̑sъ →Every word from Proto-Slavic kǫsati →Every word from Proto-Slavic kǫsъ →