That is a humorous way of contrasting the jealous patriotism of the Scot with the passionate individualism of the celt.
It is possible to note the difference in the character of the celt and Teuton.
Her courage was the blended courage of the celt, the Spaniard, and the Moor.
Because lust was not good enough, the celt invented romance.
Meredith never wrote a novel which was less a novel than celt and Saxon.
She is celt to her back-bone, with all the qualities of her race.
A celt is the child of generations of cattle-stealers, and the raiding spirit is still in the blood.
The apple seller was on deck then, and with the wisdom of the celt she understood.
As the individual celt was easily turned into a soldier, so a tribe of celts was easily turned into a battalion of soldiers.
It is quite likely that the celt itself was simply a development of the wedge.
"stone chisel," 1715, from a Latin ghost word (apparently a misprint of certe) in Job xix:24 in Vulgate: "stylo ferreo, et plumbi lamina, vel celte sculpantur in silice;" translated, probably correctly, in KJV as, "That they were graven with an iron pen and lead in the rock for ever." But assumed by others to be a genuine carving tool, partly because it was in the Bible, and thereafter adapted by archaeologists as a name for a class of prehistoric implements.
also Kelt, c.1600, from Latin Celta, singular of Celtae, from Greek Keltoi, Herodotus' word for the Gauls (who also were called Galatai). Used by the Romans of continental Gauls but apparently not of the British Celtic tribes. Originally in English in reference to ancient peoples; extention to their modern descendants is from mid-19c., from French usage.