"The irregular alternation of Pre-Gm. *epr, Pre-It. *apr and Pre-Slav. *uepr- could be an indication that the word was not inherited from PIE, but rather adopted from an unknown (extinct) source by the individual daughter languages after their settlement in Europe" (GK).
Michiel de Vaan. Etymological Dictionary of Latin and the other Italic Languages. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
Guus Kroonen. Etymological Dictionary of Proto-Germanic. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
Robert Farren, PIE culture words collection, 2017
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