Reconstruction:Proto-Slavic/kubъ

• *kǫbъ (per Vasmer)

Attested mainly in Northern Slavic, except perhaps for dial. Macedonian куб (kub, “small shape, vessel”), Bulgarian кубица (kubica, “type of gadget or toy”). No certain relatives outside of Slavic:

• Per Vasmer: Possibly related to Sanskrit कुम्भ (kumbha, “pot”), Avestan 𐬑𐬎𐬨𐬠𐬀 (xumba, “pot”) or Ancient Greek κύμβος (kúmbos, “vessel”), implying a nasal reflex (as if from *kǫbъ).
• Per Trubačev: Without nasalization, as evident by Polish kubek, Lower Sorbian kub. Possibly akin to Lithuanian kaũbras (“mound”), Phrygian Κύβελον (Kúbelon, “mountain in West Anatolia”), perhaps from Proto-Indo-European *kewbʰ- (“to arch, to bow”).

*kubъ m (North Slavic)

1. type of small container: mug, beaker, goblet

• *kuba f

• *kubъkъ, *kubikъ, *kubьcь (diminutives)
• *kubrъ (“type of toy”)
• *kubanъ (“clay pot”)
• *kubovъ (adjective)
  • *kubovina (“cleft”)

• *kъbьlъ (“bucket”) (possibly a Germanic borrowing)
• *čubъ (“crest”)

• East Slavic:
  • Old East Slavic: кубъ (kubŭ), кубъкъ (kubŭkŭ) (attested around 13-14 cent.)
    • Russian: куб (kub) (dated), ку́бок (kúbok)
    • Ukrainian: куб (kub, “wooden dish”), ку́бок (kúbok)
• West Slavic:
  • (possibly) ⇒ Czech: kuba (“type of dish”) (dialectal)
  • ⇒ Polish: kubek (“bowl, cup”) (diminutive)
  • Lower Sorbian: kub (“bowl, cup”)

• Trubachyov, Oleg, editor (1987), “*kubъ, *kubъkъ, *kubikъ”, in Этимологический словарь славянских языков [Etymological dictionary of Slavic languages] (in Russian), numbers 13 (*kroměžirъ – *kyžiti), Moscow: Nauka, page 77
• Vasmer, Max (1964–1973) “куб”, in Oleg Trubachyov, transl., Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress
• “kaubrė̃”, in Lietuvių kalbos etimologinio žodyno duomenų bazė [Lithuanian etymological dictionary database], 2007–2012