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.)", 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 of the three peoples, being farthest removed from the highly developed civilization of the Roman Province, least often visited by merchants with enervating luxuries for sale, and nearest to the Germans across the Rhine, with whom they are continually at 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."
Some translators of Caesar have given crucially different interpretations of his meaning in another passage on the Belgae. W. A. McDevitte and W. S. Bohn (1869) rendered the Latin of Caesar in Bello Gallico, II.4 as "When Caesar inquired ... he received the following information: that the greater part of the Belgae were sprung from the Germans, and that having crossed the Rhine at an early period, they had settled there, on account of the fertility of the country". A more modern translation gives the same section as: "the envoys stated that most of the Belgae were descended from tribes which long ago came across the Rhine from Germany and settled in this part of Gaul on account of its fertility."
So Caesar's use of the word "Germani" needs special consideration. He uses it in two ways. He describes 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 any tribe considered to be of similar ancestry and traditions, with ancestry east of the Rhine. So the Germani amongst the Belgae were called Germani cisrhenani, to distinguish them from other Germani, such as those living on the east of the Rhine, in the presumed homeland of the Germani. The later historian Tacitus was informed that the name Germania was recent in his day. "The first people to cross the Rhine and oust the Gauls, those now called Tungri, were then called Germani. It was the name of this nation, not a race, that gradually came into general use. And so, to begin with, they were all called Germani after the conquerors because of the terror these inspired, and then, once the name had been devised, they adopted it themselves." In other words, the collective name Germani had first been used by the Gauls or Belgae for the intruders from beyond the Rhine, and was later adopted as a collective name by the Germani themselves.
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 but that 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.