The surname Zilahi is of Hungarian origin. The root of the name “Zilah” is the town of Zilah which is located in Romania, Transylvania, it was a medieval town of the Szekler region of Transylvania and it was an important economic center. The Hungarian word “Zilahi” is a toponymic name, indicating someone from Zilah.
It was likely used as a nickname for someone who came from Zilah or someone who had ancestors from that town.
It’s worth noting that the spelling of the last name may be different in some countries, such as Zilahi, Zilahy or Zilahyi.