In Slavic mythology, Perun is the highest god of the pantheon and the god of thunder and lightning. He rides in a chariot pulled by a goat buck and carries a mighty axe, that he hurls at evil people and spirits. Perun is strongly correlated with the near-identical Pērkons from Baltic mythology. However, all the rest is conjecture.

Perun - the Slavic God of 'Thunder and Lightening'.

It has been suggested that the name Perun is a derivative of the Proto-Indo European thunder god whose original name has been reconstructed as Perkwunos. The meaning of this conjectured word Perkwunos is unknown and it is estimated that the root 'perkwu' originally probably meant 'oak', which is totally unrelated to 'Perun'. To sort that out it is said that 'perkwu' but in Proto-Slavic this evolved into per- meaning "to strike, to slay" in Proto-Slavic. But this is a long haul and too contrived.

The most obvious derivation of 'Perun' is from the Sanskrit 'Varuna' (वरुण). 'Varuna' is the Vedic God of 'Thunder and Lightening'. He is the God of the Oceans and Seas'.He is the god of all objects pertaining to water In Sanskrit the syllable 'v' () means 'wind', 'water' and 'ocean'. That explains the first syllable of the word. The second part is 'aruNa' (अरुण) and means 'ruddy', 'sun' or 'gold'. 

Varuna is mentioned in the Rig Veda - the most ancient surviving treatise of the world. Varuna, is also mentioned in the Valmiki Ramayana - as the God of the waters, who's abode are the oceans. (Yuddha Kanda, Sarga 19).

Varuna - the Vedic God of
'Lightening and Thunder, Oceans and Seas'.
Here is a Suktam from the Yajur Veda in ode of Varuna: