c. 1600, "Low Germany and the Netherlands," from the Latin name of the territory occupied by the Belgæ, a Celtic or Celto-Germanic tribe that in Roman times occupied the area below the mouth of the Rhine, including modern Belgium and much of northeastern France. Adopted 1830 as the name of a new nation formed from the southern part of the former United Kingdom of the Netherlands.
<a href="https://www.etymonline.com/word/Belgic">Etymology of Belgic by etymonline</a>
Harper, D. (n.d.). Etymology of Belgic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Belgic