Reconstruction:Proto-Tungusic/ʒapkun

• *ӡabkun (due to reason that the change *-bk- > *-pk- is more natural rather than *-pk- > *-bk-)
• *ǰapkon (per Doerfer & Knüppel) ^[1]

Ramstedt proposes two etymologies:

1. *ʒab- is identified with Evenki дябдар (ʒabdar, “long”) (see PTg entry *ʒabda), while the second component has to be borrowed from Sino-Korean word "kon" ("eldest (brother)"); and Ramstedt supposes the following semantic development: ("long brother") > ("long finger") > ("middle finger") > ("eight, 8")
2. *ӡ-ap-kan < *ӡu(r)-ap- (“2 before [10]”) in analogy with Korean 여덟 (yeodeol) < Proto-Koreanic *yər-tur-ap (“2 before 10”);

Benzing proposes an earlier form *ʒapkuan due to reason that the elements *-guan and *-gun used to form numerals.

*ʒapkun

1. eight

• Jurchenic:
  • Jurchen: 剳空 (jakun /⁠ja-kung⁠/)
    • Manchu: ᠵᠠᡴᡡᠨ (jakūn)
• Tungusic:
  • Northern Tungusic:
    • Even: дяпкан (ʒapkan)
    • Evenki: дяпкун (ʒapkun)
    • Oroqen: dʒapkʊn
    • Negidal: ӡапкун (ӡapkun)
    • Solon: jahong
  • Southern Tungusic:
    • Central:
      • Oroch: дяппу(н) (ʒappu(n))
      • Udihe: ӡакпу (ӡakpu)
    • South-Eastern:
      • Nanai: дякпон (ʒakpon)
      • Orok: ӡакпу (ʒakpu), ӡапку (ʒapku)
      • Ulch: дякпу(н) (ʒakpun)

• Benzing, Johannes (1955) Die tungusischen Sprachen. Versuch einer vergleichenden Grammatik (Abhandlungen der Geistes- und Sozialwissenschaftlichen Klasse; 11) (in German), Wiesbaden: Verlag der Akademie der Wissenschaften und der Literatur in Mainz in Kommission bei Franz Steiner Verlag, page 101
• Cincius, V. I. (1975) Сравнительный словарь тунгусо-маньчжурских языков [Comparative Dictionary of Tungus-Manchu Languages] (in Russian), volume 1, Leningrad: Nauka, page 251
• Kane, Daniel (1989) The Sino-Jurchen Vocabulary of the Bureau of Interpreters (Uralic and Altaic Series; vol. 153), Bloomington, Indiana: Research Institute for Inner Asian Studies, Indiana University, →ISBN, page 363.