Laudator Temporis Acti

Jim Dobson, "Inside The World's Largest Private Apocalypse Shelter, The Oppidum," Forbes (November 5, 2015), p. 3:

The Oppidum was created by Jakub Zamrazil, a Czech entrepreneur with a successful track record in real estate development, sales and marketing. When Mr Zamrazil first toured the unique former military facility, he was impressed by its massive scale. He saw its potential to be transformed into the ultimate life-assurance solution and named it The Oppidum, from the Latin word meaning the main settlement in an administrative area of ancient Rome. The word was derived from the earlier Latin op-pedum, an "enclosed space", used to describe fortresses that were constructed in Europe as early as the Iron Age.

Oxford Latin Dictionary, s.v. oppidum, says that the etymology of the word is "[dub.]," i.e. dubium, uncertain, and gives alternate spellings from the Corpus Inscriptionum Latinarum:

• opped- (1.583.31)
• oppod- (1.585)
• opid- (1.1529.10, 2269.3)

I haven't seen Homenaje a Antonio Tovar. Ofrecido por sus discípulos, colegas y amigos (Madrid: Editorial Gredos, 1972), but a review by Christian Devos, L'Antiquité Classique 47.1 (1978), pp. 400-401 (at 400) summarizes one of the essays in the Festschrift as follows:

Pour E. PERUZZI, lat. oppidum vient de *oppedum, à rapprocher de pedus ou pedum (Virg., Buc., V, 88), bâton fourchu destiné à entraver les animaux, notion assez voisine de celle du «box» qui retient les chevaux avant le départ de la course. Lat. *pedulum que suppose it. pieglio (à comprendre comme un «pieu, muni de branches latérales, auquel les bergers suspendent leurs ustensiles»), manifestement diminutif de pedus ou pedum serait une survivance de la même racine.

I don't have access to Michiel de Vaan, Etymological Dictionary of Latin and the Other Italic Languages (Leiden: Brill, 2008). See Alfred Ernout and Alfred Meillet, Dictionnaire Étymologique de la Langue Latine. Histoire des Mots, 4th ed. (Paris: Klincksieck, 2001), p. 463:

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Thanks to a dear friend for letting me see the entry on oppidum in de Vaan's dictionary (pp. 430-431):