There is an interesting Bulaestian phraseological locution “мокрей як щур” (//|мокрей йак ш’: ур//). In a word by word translation it means ‘wet as a rat’; it is spoken about a person who gets wet in a heavy rain. As a variant (but rarer) Bulaestian dialect’s speakers use “мокрей як меш” (//|мокрей йак меш//), i. e, ‘wet as a mouse’. The adequate translation of both variants in English has to give the meaning of “completely wet; soaked to the skin”. So, what is the origin of the idiom? The Ukrainian special dictionaries include neither “мокрий як щур”, nor “мокрий як миш”. The meaning of “completely wet; soaked to the skin” is conveyed by some other idioms in the Ukrainian language. However, “мокрий як щур” is known in an Ukrainian dialect (the Krasilov district of the Khmelnitsk region; I am thankful to N. D. Kovalenko and I. V. Gorofyanyuk for the information). Thus, we can’t exclude that it exists in some other Ukrainian dialects as well. But, it is obviously a very rare and archaic idiom. Well, the Russian language has the idiom “мокрый как мышь” with exactly the same meaning of “completely wet; soaked to the skin”. The idiom “mokry kaž myš”, identical to Russian “мокрый как мышь”, is also known in the Upper Lusatian language. It makes us suppose that the idioms have a for-Slavic origin. Slavic *ščиrъ ‘rat’ is absent in the Russian language. But, there is a well-known Russian щурёнок ‘young pike (Esox lucius)’ (to the letter: ‘small щур’). Thus, we can suppose the existence of Russian щурёнок ‘young pike (Esox lucius)’ as a reason for the appearance of Slavic idioms like “мокрей як щур”/“мокрей як меш”. Some ancient intercrossing of semantics between different Slavic lexemes *ščиrъ (and *aščerъ ‘lizard’, also) could provoke the appearance of some new meanings. And, we know for sure that such ancient intercrossing of semantics happened.