I just happen to come across this link while searching for references to "Murkys" / "Mourquis".
So with respect to the start of this page more than 11 years ago I am aware of the likelihood that everybody having contributed to this thread has solved the question for her/himself long since. So, please forgive me being smart when just sharing my own clarifications, which to some extent may appear repetitive (e.g. Gitta, e.g. Grishka).   
I came across the term when reading in Carl Philipp Emanuel Bach's "Versuch über die wahre Art Klavier zu spielen" (Essay on The Correct Way of Playing The Piano/clavicord). In §5 of his introduction he criticizes >... the torturing of students with tasteless Murkys, where the left hand is solely used for a noisy play thus being made forever unfit for its true use ..... < The term "Murkys" then is commented on by a remark of the editor which translates as follows: >Accompaniment consisting of continuously broken oktaves or likewise whole pieces on such unartistic bass (pl.) - in earlier piano music had been called Murkys (Murkybasses) or "Mourquis".< An example of notes is given with this remark: it is a sequence of sixteenth notes continuously jumping from the bass G (lowest line of the bass clef) to the G one octave up (between the upper 2 lines of the bass clef). "broken octave" here (differing from its use with regard to intervals) refers to subsequent striking (=appregio) as opposed to simultaneous playing of a chord. The result is a very primitive accompaniment like "hwoomb-da-hwoom-da" as it is well known from all sorts of traditional folk music, namely todays German folk music. Well, I believe many of us have memories of their own little attempts of accompaniment with their first fun-approaches to the piano. And father Leopold Mozart may have composed that "Mouqui" for little Wolferl for the same reason.
Like Grishka I had to think of the German word "Murks" (engl. 'botch'), although I took it the other way around: could it possibly be that "Murks" derives from "Murkys", expressing that something has been carried out pretty unprofessionally. The verb is "vermurksen". Then there is also "abmurksen", meaning 'to kill sb' or sth, e.g. a car's engine. According to some ethymological sources there was also "murken" (extinct) = 'murdering', and its supposed iterative "murksen". But such tracings appear to be poorly founded and for that matter arguable to me.
Sorry, I hope I was able to make myself understood, as I am just another German speaker, plus I am not a musician.