Many thanks to Alan Saxby for pointing this little story out:
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 present 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 (1935), pp. 27-28, n. 1
We’re now told (I think, correct me if wrong) that Weiss was the first to use Q but still…the above would be a much more entertaining myth of origins, would it not?
UPDATE: it seems Eduard Simons was using the abbreviation ‘Q.’ in 1880 and Weiss in 1890. Thanks to Mark Goodacre for bibliography. So, as Deane and Steph imply in the comments, the question really is this: what was Robinson thinking…?