ENthe baltic and the Slavic languages evolved in parallel and influenced each other significantly. one area of mutual influence concerns pre-christian vernacular traditions. among the most important supernatural entities common to both traditions, and reflected in linguistic data, is the thunder god (lithuanian Perkūnas, latvian Pērkons, pol. Piorun, old russian Перунъ). even if the names of the two thunder gods appear to be etymologically unrelated, their semantic development followed a similar path. the aim of this article is to analyze the phraseological units associated with the thunder gods in baltic (lithuanian and latvian) and Slavic (polish and “belarussian”) in order to identify possible similarities and differences. the results seem to indicate that the power of the thunder god is invoked in curses and swearword, following the vernacular myth of thunder striking an evil entity. this interpretation is strengthened by the data contained in the folkloric beliefs. With the christianisation in polish language the thunderstrike became the instrument of god and at the same time Piorun assumed the connotation of the devil, while in lithuanian Perkūnas appears to have been assimilated as a subordinate of the christian god. the ambiguity of the expressions where thunder and thunderstrike are present helped their durability, as it not always clear when these entities or phenomena are seen as positive or negative. Keywords: phraseology, etymology, Perun, Perkūnas, mythology, ethnolinguistics.