The Belgae were well known on the Continent as a group of tribes living in the north of Gaul, in the province that the Romans called Gallica
Belgica. In Britain they are known only from Venta Belgarum (Winchester) listed by AI and RC, and from Ptolemy’s mention of Βελγαι living south οf the Δοβυννοις and having settlements Ισκαλις (probably the lead mines at Chartherhouse), Υδατα Θερμα ‘hot waters’ (Bath), and Ουεντα (Winchester). No one has adequately explained how
Continental Belgae could have ended up that far west in Britain.
The name probably derives from PIE *bhelgh- ‘to swell, bulge’. Modern Belgium has a relatively recently created name. Its Flemish inhabitants point to the difficulty Caesar had in subjugating the northern tribes (who claimed affinity with fierce and explicitly Germanic tribes across the Rhine) and therefore point to descendant words such as Old English belgan ‘to enrage oneself’. Its Walloon inhabitants (whose language descends from Latin) are more willing to believe the theory that most Belgae were Celtic speakers, and therefore to point to words such as Old Irish bolg ‘bag, belly’ as parallels.