From Middle High German zūn, from Old High German zūn, from Proto-Germanic *tūną (“enclosure”). Cognate with German Zaun (“fence”), Dutch tuin (“garden”), English town.
Since Luxembourgish velarises Middle High German -ūn-, -iun- only in open syllables, the expected nominative singular would be *Zaun. The form Zonk is a backformation from the dative singular *Zong by analogy with such words as Spronk (from Middle High German sprunc, dative sprunge). Eventually the final -k was generalised also in the plural; likely to avoid homophony with zéng (“ten”). The Rheinisches Wörterbuch still attests singular Zonk with plural Zéng for the region of Bitburg.
Zonk m (plural Zénk)