Instructivity of the ethno-linguistic approach to etymology, especially when dealing with the archaic mythological vocabulary, has been shown already in the works of O. N. Trubachev, V. N. Toporov, V. V. Ivanov, N. I. Tolstoy and S. M. Tolstaya and others. The article considers a term of the mythological vocabulary denoting the "walking" deceased, known to all Slavic languages – upyr’ ‚ghoul‘. The study gives a brief overview of the existing etymological variants of this word, which has no unambiguous interpretation as of yet. It also comments — in terms of traditional Slavic folk culture — on the semantics of upyr’ as an ‘unburned deceased‘, or a flying but ‘non-bird‘ creature, or the one ‘flying up from the grave‘, or a mythological character who sticks into the victim, etc; as restored by some etymologists. A possibility of the term being borrowed is also discussed. The article offers another etymological interpretation for upyr’ based on the verb *piti ‘to drink‘ with a prefix ǫ- and a suf- fix -r-). This version is eventual due to the cultural semantics of this mythological character: (1) as a being who drinks up vital force (and its substance, blood) of humans and animals, robs them of life and thereby gains extra life for him-/herself; (2) as an unholy, ever-athirst dead and (3) as a ghost of a deceased one who possessed during his/her lifetime a greater amount of energy than an ordinary human.