The traditional etymology is from the name of a legendary founder, кии (kii), mentioned in the Hypatian Codex (see Kyi, Shchek and Khoryv in Wikipedia). Compare Ukrainian Кий (Kyj), Russian Киев (Kíjev), from Кий (Kij). However, historian Michael Hrushevsky warned that this was an “etymological myth”.
A proposal by linguist Jaroslav Rudnyckyj holds that Київ comes from кий (kyj, “stick, pole”), meaning a settlement palisaded киями (kýjamy, “by poles”). This etymology has been accepted by several émigré linguists, and supported by multiple lines of evidence. In turn, the word comes from Proto-Slavic *kyjь (“pole, hammer”), from Proto-Indo-European *kū-i̯os (“pole, hammer”), *kāu̯-, *kəu̯-, related to Proto-Indo-European *kūti (“to hit”), from *koutei.
- Melʹnyčuk O. S., editor (1982–2012), “кий”, in Etymolohičnyj slovnyk ukrajinsʹkoji movy [Etymological Dictionary of the Ukrainian Language] (in Ukrainian), Kiev: Naukova Dumka
- Rudnycʼkyj, Jaroslav B. (1982), “Київ”, in An Etymological Dictionary of the Ukrainian Language (in Ukrainian), volume II, Ottawa: Ukrainian Mohylo-Mazepian Academy of Sciences, Ukrainian Language Association, pages 660–667