Reconstruction:Proto-Slavic/sverěpъ

• *svirěpъ

From earlier *(e)svorěpъ, from *esva (“horse”) +‎ *rěpъ (“bur, burdock”). For the loss of the initial vowel, cf. Old Prussian sweikis (“plowhorse”) (from *asveikīs); according Machek it was assimilated under influence of -ě-. Such etymology is supported by the dialectal Czech equivalent koňský řepíček "horse's Agrimonium", Romanian borrowings from Church Slavonic: sireáp and svireáp (“rampant (on a horse)”) and Czech and Old Polish descendants meaning "stallion", "mare"[1].

According to Brückner: from *svarъ + *-ěpъ[2].

*sverěpъ m[1]

1. burdock
2. (by extension) wild plants

• *sverěpa f

• East Slavic:
  • ⇒ Ukrainian: свирі́па (svyrípa, “Beta, Erysimum”)
  • ⇒ Russian: свирепа (svirepa, “Raphanus”), свире́пка (svirépka, “Beta, Erysimum”)
• South Slavic:
  • ⇒ Serbo-Croatian:

    Cyrillic script: сверепек (“Festuca, Aegilops”)

    Latin script: sverepek (“Festuca, Aegilops”)

• West Slavic:
  • Old Czech: sveřěp (“draff”)
    • ⇒ Old Czech: sveřěpicě f (“mare”)
    • Czech: sveřep (“Bromus”)
      • ⇒ Czech: sveřepec (“breeding stallion”)
  • ⇒ Old Polish: świerzepa (“mare”), świerzopa (“mare”), świerzepic (“stallion”)
    • Polish: świerzop (“Raphanus”), świerzepa (“Sisymbrium”), świrzepa (“Rapistrum”)
    • → Old Prussian: sweriapis (“stallion”)
  • ⇒ Old Slovak: sverepa f, sverepica f, sverepec m (“Bromus”)

Unclear:

• Machek, Snoj: From *sve- (“self”) +‎ *rěpъ (“clinging”), originally meaning “one who relies on themselves”.
• Miklošič, Fortunatov: Akin to Lithuanian šiur̃pti (“to bristle, to shiver”).
• Mladenov: Related to Lithuanian sver̃ti (“to scale, to weigh”), svìrtis (“lever”), svarùs (“important”), Latvian svērt (“to weigh”), Proto-Germanic *swēraz (“heavy”) (effectively from Proto-Indo-European *swer- (“grave”) + *-ěpъ). Potebnja also relates dial. Russian осве́р (osvér, “lever arm”), possibly сверёжой (sverjóžoj, “wayward”) to this set of cognates.
• V. Merkulova: From the root of *sur- (“raw”) +‎ *-ěpъ.
• Holzer: Possibly derived from a non-Slavic IE substrate (again + *-ěpъ), cognate with native *zvěrь (“beast”).

The origin of the elusive suffix *-ěpъ in the last three proposals is uncertain. Found also in dialectal terms such as *kъrlěpъ (“sticky”) (> dial. Serbo-Croatian крлијеп, stand. крпељ (“tick”) (via metathesis)), *skorěpъ (“hollow”) (> Czech skоřера (“husk”)).

Vasmer, Brückner, Jagić, Blažek discuss the adjective alongside the noun, (presumably) implying kinship between the two.

*sverěpъ[1]

1. fierce, vicious
2. wild, savage

• *sverěpostь, *sverěpota (abstract nouns)

• East Slavic:
  • Old East Slavic: сверѣпъ (sverěpŭ), свирѣпъ (svirěpŭ)
    • Russian: свире́пый (svirépyj)
      • → Serbo-Croatian:
        • Cyrillic script: свѝреп
        • Latin script: svìrep
    • ⇒ Ukrainian: свірі́пний (svirípnyj)
• South Slavic:
  • Old Church Slavonic:

    Old Cyrillic script: сверѣпъ (sverěpŭ)

    Glagolitic script: ⱄⰲⰵⱃⱑⱂⱏ (sverěpŭ)

    • → Romanian: sireáp, svireáp (archaic)
  • Bulgarian: свире́п (svirép)
  • Macedonian: свиреп (svirep)
  • Slovene: svеrе́р, srẹ̑p (tonal orthography)
• West Slavic:
  • Old Czech: sveřěpý
    • Czech: sveřepý
  • Old Polish: świerzepi, świerzepy, świerzopi, świerzopy
  • Slovak: sverepý

• Vasmer, Max (1964–1973) “свире́пый”, in Oleg Trubachyov, transl., Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress
• Racheva, M., Todorov, T. A., editors (2002), “свиреп”, in Български етимологичен речник [Bulgarian Etymological Dictionary] (in Bulgarian), volumes 6 (пỳскам – словàр²), Sofia: Prof. Marin Drinov Pubg. House, →ISBN, page 553
• “šiurpti”, in Lietuvių kalbos etimologinio žodyno duomenų bazė [Lithuanian etymological dictionary database], 2007–2012