Reconstruction:Proto-Slavic/kubъ

• *kǫbъ (per Vasmer)

Attested mainly in East and West Slavic, with no certain relatives. South Slavic preserves the term Old Church Slavonic къбьлъ (kŭbĭlŭ, “bucket”) (encountered also in East Slavic, probably identical to Polish kubeł), metathemed in Late Church Slavonic as бъкълъ (bŭkŭlŭ), бъклица (bŭklica) under the influence of Ancient Greek βαύκαλις (baúkalis) or Vulgar Latin baucalis.

There are two lines of thought:

• Vasmer speculates towards a relation with Sanskrit कुम्भ (kumbha, “pot”), Avestan 𐬑𐬎𐬨𐬠𐬀‎ (xumba, “pot”) or Ancient Greek κύμβος (kúmbos, “vessel”), implying a nasal reflect (as if from *kǫbъ).
• Trubachev objects against this hypothesis in view of Polish kubek, Lower Sorbian kub which have no sign of nasalization. Instead, a regular *-u- reflex is proposed.

Per Trubachev, likely from Proto-Indo-European *kewbʰ- (“to duck, to bow”) attested in Ancient Greek κοῦφος (koûphos, “light, hollow”), Ancient Greek κουφάριν (kouphárin, “carcass”) or possibly Proto-Indo-European *kewb- (“to bend, to curve”) whence are Proto-Germanic *huppōną (“to hop”), Ancient Greek κύβος (kúbos, “cube”) (from where are the late loanwords Russian куб (kub), Bulgarian куб (kub), etc via Latin cubus). If Old Church Slavonic къбьлъ (kŭbĭlŭ), Polish kubeł are indeed related, then a borrowing/back-formation from Latin cupa via Old High German kūbel is also plausible.

*kubъ m

1. drinking vessel, bucket

   Synonyms: *vědro, *čьbьrъ

2. (by extension) tank

• *kъbьlъ (“bucket”) (possibly)
• *kubьlъ (“bucket”) (possibly)
• *čubъ (“forelock, crest”) (possibly)
• *čubьrica (“summer savory”) (possibly, also attested with a nasal reflex: *čębьrъ)

• *kubъkъ, *kubьcь (diminutive)
• *kubikъ (“wooden toy”)
• *kubarь (“type of toy”)
• *kubanъ (“clay pot”)
• *kubovъ
  • *kubovina (“cleft”)

• East Slavic:
  • Old East Slavic: кубъ (kubŭ)
    • Russian: куб (kub) (dated), кубок (kub-ok)
    • Ukrainian: куб (kub, “wooden dish”)
• West Slavic:
  • (possibly) ⇒ Czech: kuba (“type of dish”) (dialectal)
  • ⇒ Polish: kubek (“bowl, cup”) (diminutive)
  • Lower Sorbian: kub (“bowl, cup”)

• Vasmer, Max (1964–1973), “куб”, in Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), transl. and suppl. by Oleg Trubachyov, Moscow: Progress
• Trubachyov, Oleg, editor (1987), “*kubъ, *kubъkъ, *kubikъ”, in Этимологический словарь славянских языков [Etymological Dictionary of Slavic Languages] (in Russian), issue 13 (*kroměžirъ – *kyžiti), Moscow: Nauka, page 77