None of the hitherto proposed attempts at explaining the origin of the East-Slavonic numeral sorok ‘40’, is satisfactory. This refers almost equally to those propositions that derive it from Greek (F. Miklosich, A. Brückner, V. Jagić), Turkish (O.N. Trubačëv) or from Old Norse (M. Vasmer). The author of the current article puts forward a yet another solution, this time pointing to the Ugro-Finnic languages. As the basis of the borrowing, the Udmurt proto-form *śor-ku/*sor-ku is advanced, which was adapted to *sork > sorok on the East-Slavonic ground. A possible semantic evolution leads from ‘marten pelt(s)’ > ‘a bundle of pelts’ > ‘a bundle of pelts of forty pieces (as many as was needed to sew one fur-lined overcoat’ > ‘a numerical unit (measure) used in trade’ to ‘a stand-alone number 40’. The proposed etymology corresponds well with the context of Ugro-Finnic – East-Slavonic linguistic and extralinguistic contacts.