I have always taught, for I was always taught, that that the hypothetical source “Q” for material common to Matthew and Luke, took it’s name from the German word Quelle for “source”. However, this might not be the case. Over at Sheffield Biblical Studies I read this quote from R.H. Lightfoot:
It seems now to be assumed that the symbol Q originated in Germany, as being the first letter of the German Quelle, source. Dr Armitage Robinson, however, in conversation with the pesent writer maintained in all seriousness that he himself was the first to use the symbol, and for an enitrely different reason. In lecturing at Cambridge on the sources of the gospels, in the ‘nineties of the last century, he was in the habit, he said, of alluding to St Mark’s gospel as P (reminiscences of St Peter), and so the presumed sayings-document as Q, simply because Q was the next letter after P in the alphabet. His contention, therefore, was that some of his hearers carried his method across the North Sea, and that German scholars, having adopted the symbol Q from him, soon found an explanation for it, which to them no doubt seemed both more satisfactory and more rational. Dr Robinson emphasized that no designation of the sayings-document by the symbol Q appeared in German writings until after the period of his lectures at Cambridge,and that the now common explanation of the symbol would be found to be still later. If, as Dr Burkitt informs me, Wellhausen was the first in Germany to use the symbol Q, it is possible to date accurately its appearance in print in that country, since the first edition of his Einleitung, in which it appears, was published in 1903. His commentaries on the synoptists began to appear in the same year. (R.H. Lightfoot, History and Interpretation in the Gospels, 27-28, n. 1).