Tuhkru (Tuhkur):

Polecat, or common ferret. This is the wild ancestor of the ‘ordinary’ ferret. Native to Estonia is the Metstuhkur, European polecat, Mustela putorius. The name comes from old Pskov or Novgorod Russian, presumably due to Novgorod and, later, the Principality of Moscow, being prime agents in the fur trade^[1]. For example, while 20, 30, 50, 60, 70, 80 & 90 all follow a similar pattern: двадцать (dvadtsat’), тридцать (tridtsat’), пятьдесят (pyat’desyat), шестьдесят (shest’desyat), семьдесят (sem’desyat), восемьдесят (vosem’desyat) & девяносто (devyanosto) where – excluding 90 which may come from an earlier *nevenǝdḱɨ̥̄tǝ, or ‘ninth ten’ – most follow the 2-9 sequence of ‘two tens’, ‘three tens’, etc., with 0-5 digits typically more simplified than 6-9s. Forty, on the other hand, is сорок (sorok, from Old Norse serkr, ‘shirt’, but word rubbing shoulders with ‘skin’), probably comes from the fur trade where Old Russian sorokъ represented “a bundle of 40 sable skins” 1 of which = 5 timbrs, and 1 timbr = 40 skins. Other 40-based ‘skin’-related terms include old Slovak meru, 40 (now štyridsať), which may have developed according to similar principles as сорок from Hungarian mérő, ‘bag’ or ‘sack’ (but this itself is also suggested as deriving from Old Slavonic (?), taking us back to square one), known to be used as a measure of grain, but could also be a remnant of the fur trade. As an aside, the pre-euro Croatian currency kuna (marten, another mustelid related to the ferret) of 1994-2023 was named in reference to the skin’s usage as currency in the middle ages, and Ukraine’s crypto Exchange is also called Kuna. The question may be a trifle more complex than this^[2].

^[1] See Que fait le français après soixante-neuf ? (forthcoming, one day, inchallah…) somewhere in www.frogologue.com

^[2]  Patient French-speaking readers with a life expectancy of 30+ years may sign up for alerts.