An anonymous writer on the Sheffield Biblical Studies blog has a nice post on Why was ‘Q’ named ‘Q’: because ‘Q’ comes after ‘P’? in which s/he quotes R. H. Lightfoot's contention that the symbol "Q" did not originate in Germany but rather in England. J. Armitage Robinson claimed that he used it in the 1890s as the letter coming after "P" in the alphabet. For him, "P" stood for "Peter", the alleged source of Mark's Gospel.
It's a great myth of origins and while it is quite possible that Robinson introduced the term "Q" independently of the German scholarship, there is no question that the term is in fact first used in the German scholarship. Frans Neirynck shows that the use of "Q." as an abbreviation for Quelle goes back to Eduard Simons, Hat der dritte Evangelist den kanonischen Matthäus benutzt (Bonn: Universitäts-Buchdruckerei von Carl Georgi, 1880). It is then used as "Q", without the dot, from 1890 onwards, by Johannes Weiss, and with full critical self-awareness by Paul Wernle in 1899. Some bibliography:
Frans Neirynck, "The Symbol Q (=Quelle), ETL 54 (1978): 119-25
Frans Neirynck, "Once More: The Symbol Q", ETL 55 (1979): 382-3
Frans Neirynck, "Note on the Siglum Q", Evangelica II (BETL 99; Leuven:
Leuven University Press, 1991), 474
See too the helpful summary in:
James M. Robinson, Paul Hoffmann and John S.
Kloppenborg. The Sayings Gospel Q in Greek and English: with Parallels from
the Gospels of Mark and Thomas (Minneapolis, MN: Fortress, 2002), 23-4