Contrary to other gods of the Proto-Indo-European pantheon such as Dyēus, the sky-god, or Hausōs, the dawn-goddess, widely accepted cognates stemming from the root *Perkwunos are only attested in Western Indo-European traditions. The linguistic evidence for the worship of the thunder-god Perkwunos as far back as Proto-Indo-European times (4500–2500) is therefore less secured.
The term *Perkwunos likely stems from the verbal root *per-, "to strike". Another proposed etymology is the root *pérkʷus, the oak, attached to the divine nomenclature *-nos, "master of", with cognates in Latin quercus ("oak-tree"), Germanic *ferhwa ("oak"), Gaulish erc-, ("oak"), and Greek Herkyna (a spring-nymph associated with the Herkyna River).
His name thus either meant "the Striker" or "the Lord of Oaks". An etymology uniting those two propositions has been suggested in the mythological association of oaks with thunder, explained by the frequency with which such tall trees are struck by lightning, although this is not universally agreed upon.
Other cognates related to thunder, through another root *(s)tenh₂, are found in Germanic Thor, Celtic Taranis and Latin (Jupiter) Tonans. According to Jackson, "they may have arisen as the result of fossilisation of an original epithet or epiclesis", as the Vedic god Parjanya is also called stanayitnú- ("Thunderer").
George E. Dunkel regarded Perkwunos as an original epithet of Dyēus, the Sky-God. It has also been postulated that Perkwunos was referred to as *Diwós Putlós ("son of Dyēus"), although this is based on the Vedic poetic tradition alone.
Perkwunos is depicted as holding a weapon, generally conceived as a club, mace, or hammer, made of stone or metal. In the Latvian poetic expression Pērkōns met savu milnu ("Pērkōn throws his mace"), the mace milna is cognate with the Old Norse mjollnir, the hammer thrown by the thunder god Thor, but also with the word "lightning" in Old Prussian mealde and Old Church Slavonic *mlъni. His thunder and lightning had a destructive connotation, but they could also be seen as regenerative since they were escorted by fructifying rain. Parjanya is depicted as a rain god in the Vedas, Latvian prayers included a call for Pērkōns to bring rain in time of drought, and the Balkan Slavs worshipped Perun along his female counterpart *Perperuna in prayers calling for vital rains, which portrayed the female as a virgin who had not yet had her first monthly period. The earth is likewise referred to as "menstruating" in a Vedic hymn to Parjanya, a possible cognate of *Perperuna.
Oak-God and Striker
The association of Perkwunos with the oak is attested in various formulaic expressions from the Balto-Slavic languages: Lith. Perkūno ąžuolas (Perkūnas's oak), Latv. Pērkōna uōzuōls ("Pērkōn's oak"), or ORus. Perunovŭ dubŭ ("Perun's oak"). The Slavic thunder-god Perūn is said to frequently strike oaks to put fire within them. In Germanic mythology, Thor likewise strikes his foes the giants when they hide under an oak; although he is a thematic echo and not a linguistic cognate of Perkwunos. The striking of devils, demons or evildoers by Perkwunos is a motif also encountered in the myths surrounding the Baltic Perkūnas and the Vedic Parjanya.
A multi-headed water-serpent is connected in particular with Perkwunos in an epic battle: the serpent is often described as a "blocker of waters", and his heads eventually smashed by the thunder-god to release torrents of water that had previously been pent up. The myth has numerous reflexes in mythical stories of battles between a serpent and a god. The latter is not necessary etymologically related to Perkwunos, but is always associated with thunder or drought: the Vedic Vṛtra (the personification of drought) and Indra, the Iranian Tištry/Sirius and Apaoša (a demon of drought), the Albanian Kulshedra (an amphibious serpent who causes streams to dry up) and Drangue, the Greek Typhoeus and Zeus, or the Germanic Miðgarð and Thor.
Perkwunos is often portrayed in connection with stone and (wooden) mountains, probably because the mountainous forests were his realm. A cognate keening appears in Germanic *fergunja ("[mountainous] forest") and Gaulish (h)ercunia ("[oaks] forests"). Words from a root *pér-ur are also attested in Hitt. pēru ("rock, cliff, boulder"), Avestan pauruuatā ("mountains"), and Sanskrit párvata ("rocky, cliff, mountain"). In Germanic mythology, Fjörgynn's name was used as a poetic synonym for "land" or "the earth", and she could have been the mistress of the wooded mountains, the personification of what appears in Gothic as fairguni. Additionally, the Slavic Perūn sends his axe or arrow from a mountain or the sky, and the Baltic tradition mentions a perpetual sacred fire maintained for Perkūnas in the forests or on hilltops.
A term for the sky, *h₂ekmōn, denoted both "stones" and "heaven". The motif of the stony skies can be found in the story of the Greek Akmon, the father of Ouranos and the personified Heaven. His name, deriving from *h₂ekmōn, meant "a block of stone or metal designed to withstand a battering". Other cognates denoting "stones" appear in Vedic áśman, Iranian asman, Lithuanian akmuõ, and possibly in Germanic *hemena. The mythological association can be explained by the observation (or delusion) that certain stones had fallen from the sky: the word áśman is used for Indra’s weapon, and the thunder-stone is called Perkūno akmuo ("Perkuna's stone") in Lithuanian tradition.
The following cognates stem from the root *Perkwunos and are associated with weather-gods in Western Indo-European mythologies:
- Baltic: *Perkūn,
- Slavic: Perūn, the thunder-god,
- Germanic: *Fergunja, "mountain", maybe "mountainous forest" (from *perwunīî), or the feminine equivalent of *ferga, "god",
- Celtic: *Ferkunyo (from *perkwun(i)yā, the "realm of Perkwunos").
Other cognates are less secured:
- Vedic: Parjanya, the god of rain, thunder and lightning (although Sanskrit sound laws rather predict a parkūn(y)a form; an original *pergénio has therefore been postulated, with a possible cognate in Slavic *per(g)ynia, "wooden hills"),
- Greek: Keraunos (κεραυνός), the name of Zeus’s thunderbolt, which were sometimes also deified (by metathesis of *per(k)aunos; although the root *ḱerh₂, "shatter, smash" has also been proposed),
- Albanian: Perëndi, a thunder and sky god, from the root *per- ("to strike") attached to the suffixes -en- and -di (*dyeus),
- Thracian: Perkos/Perkon (Περκος/Περκων), a horseman hero,
- Nuristani: Pärun/Pērūne, a war god.
Some scholars have argued that the functions of the Anatolian weather gods Tarḫunz and Tarḫunna stem from those of Perkwunos, but that Anatolians dropped the old name to adopt the epithet Tṛḫu-ent- ("conquering", from PIE *terh2, "to cross over, pass through, overcome"), which sounded closer to the name of the Hattian Storm-god Taru.
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