Origin of the word Q

1. 16th letter of the classical Roman alphabet, from the Phoenician equivalent of Heb. koph, which was used for the more guttural of the two "k" sounds in Semitic. The letter existed, but was little used and not alphabetized, in Gk.; the stereotypical connection with -u- began in Latin. Anglo-Saxon scribes adopted the habit at first, but later used spellings with cw- or cu-. The qu- pattern returned to Eng. with the Norman Conquest. Scholars use -q- alone to transliterate Sem. koph (e.g. Quran, Qatar, Iraq). In Christian theology, Q has been used since 1901 to signify the hypothetical source of passages shared by Matthew and Luke, but not in Mark; probably it is an abbreviation of Ger. Quelle "source."

via Online Etymology Dictionary, ©2001 Douglas Harper