The consensus among linguists is that the ethnic name Belgae comes from the Proto-Celtic root *belg- or *bolg- meaning "to swell (particularly with anger/battle fury/etc.)", cognate with the Dutch adjective gebelgd, "to be very angry" and verbolgen, "being angry", and the Old English verb belgan, "to be angry" (from Proto-Germanic *balgiz), derived ultimately from the Proto-Indo-European root *bhelgh- ("to swell, bulge, billow"). Thus, a Proto-Celtic ethnic name *Bolgī could be interpreted as "The People who Swell (particularly with anger/battle fury)".
Julius Caesar describes Gaul at the time of his conquests (58–51 BC) as divided into three parts, inhabited by the Aquitani in the southwest, the Gauls of the biggest central part, who in their own language were called Celtae, and the Belgae in the north. Each of these three parts was different in terms of customs, laws, and language. He noted that the Belgae, were "the bravest, because they are furthest from the civilization and refinement of Province, and merchants least frequently resort to them, and import those things which tend to effeminate the mind; and they are the nearest to the Germans, who dwell beyond the Rhine , with whom they are continually waging war". Ancient sources such as Caesar are not always clear about the things used to define ethnicity today. While Caesar or his sources described the Belgae as distinctly different from the Gauls, Strabo stated that the differences between the Celts (Gauls) and Belgae, in countenance, language, politics, and way of life was a small one, unlike the difference between the Aquitanians and Celts. The fact that the Belgae were living in Gaul means that in one sense they were Gauls. This may be Caesar's meaning when he says "The Belgae have the same method of attacking a fortress as the rest of the Gauls."
Caesar in Bello Gallico, II.4 wrote:
So Caesar's used the word "Germani" in two ways. He described a grouping of tribes within the Belgic alliance as the "Germani", distinguishing them from their neighbours. The most important in his battles were the Eburones. The other way he uses the term is to refer to those related tribes east of the Rhine, who were not Celtic. So the Germani among the Belgae are called, based on Caesar's account, the Germani cisrhenani, to distinguish them from other Germani living east of the Rhine in what he understood to be their homeland. However, the later historian Tacitus was informed that the name Germania was known to have changed in meaning:
In other words, Tacitus understood that the collective name Germani had first been used in Gaul, for a specific people there with connections beyond the Rhine, the Tungri being the name of the people living where the Eburones had lived in later imperial times, and was later adopted as a collective name for the non-Celtic peoples beyond the Rhine, the other more well-known way that Caesar used the term.
Caesar's book The Gallic Wars begins: "All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in ours Gauls, the third. All these differ from each other in language, customs and laws." However, many modern scholars believe that the Belgae were a Celtic-speaking group. On the other hand, at least part of the Belgae may also have had significant genetic, cultural, and historical connections to peoples east of the Rhine, including Germanic peoples, judging from archaeological, placename, and textual evidence. It has also been argued based on placename studies that the older language of the area, though apparently Indo-European, was not Celtic (see Nordwestblock) and that Celtic, though influential amongst the elite, might never have been the main language of the part of the Belgic area north of the Ardennes. For example, Maurits Gysseling, suggest that prior to Celtic and Germanic influences the Belgae may have comprised a distinct Indo-European branch, termed Belgian.
However, most of the Belgic tribal and personal names recorded are identifiably Gaulish, including those of the Germani cisrhenani, and this is indeed also true of the tribes immediately over the Rhine at this time, such as the Tencteri and Usipetes. Surviving inscriptions also indicate that Gaulish was spoken in at least part of Belgic territory.