Does anyone know what the meaning of "Rumgejuxe" is? I assume it is related to "juxen" or something, but I'm not sure. I googled it, but it does not have many matches.
There are a few nouns of the form Ge- + -e derived from verbs:
das Gebrüll(e), Gehabe, Gelaber(e), Geklopfe, Gelache, Gerede, Getue, Gezanke, ...
The meaning is pejorative, i.e. the activity denoted by the noun is perceived as being unpleasant.
So Gejuxe is a noun derived from juxen (to joke). The prefix (he)rum literally means around; here, it denotes an activity without a goal (as it can in English as well). Which leads us to Rumgejuxe ~ the act of joking around.
In addition to David Vogt's answer: Some of the verb + Ge constructs in popular speech combine with Rum, some not:
Verb + Ge + Rum
- Rumgejuxe (obviously)
- Rumgeschreie (Hört endlich mit dem Rumgeschreie auf!)
- Rumgedudel (typically with a musical instrument, or a radio)
- Rumgeklimper (on a piano)
- Rumgefuhrwerke (rare, rather: das Rumfuhrwerken)
- Rumgeklopfe (here start craft trades or home improvement activities)
- Rumgeschnipple (with scissors, also: Rumgeschnippel)
- Rumgesauge (with a vacuum cleaner; could also refer to a suckling)
- Rumgekaspere (difficulties to find the correct spelling show us that all these words are never used in writing)
Contrary to that I am not aware of a popular Rum prefix for
If one searches for reasons of semantical logic, I would suggest: all the Rumge- words refer to activities of a person that create noise or motion suitable to annoy others. Gehabe, in contrast, refers rather to a posture or attitude not actually emitting noise or motion.
The Rumge- double prefix is very productive. You can use it on any verb fitting the above description. So, you could try to use
if your girlfriend does not stop blow-drying her hair for hours. But I have never met that word in actual use.
Note that all these words are pretty much restricted to oral, casual communication. You would not find them in written communication (unless of course in literature depicting oral speech as in drama). Using them correctly in oral communication, however, would be an indicator of a high level of command of German. (Not least because they would be hard to find in a dictionary.)
A "Jux" is a joke or a prank. "Rumgejuxe" is the act of joking around, not necessarily in a nice way. If some people want to work seriously and others just fool around, the serious guys might say: stop that "Rumgejuxe". "Rumgejuxe" is rather colloquial.
The closer your word is to the proper, correct, precise, formal word for the action, the more respect you have for the way it's being done. If the word you use is close to, but not exactly "correct", you're just being casual (and as somebody pointed out, you're probably speaking, not writing). If you get further away, you're being disrespectful or downright insulting.
Example: your Rumgejuxe. Juxen is a correct, formal word for joking, and rum is short for herum (around), so juxen means you're joking around, casually, like with your buddies. Juxerei is another version. These are not insulting, merely casual.
Now, if you use a word that could be applicable, is more generic and further from the proper word, e.g., getue or rumgetue (doing around), it starts to get into the realm of serious disrespect. You're saying what's being done might not even be recognizable as joking, it's so bad. If you were to use geschrei (screaming), this is a flatly incorrect description of the action, and thus very far from correct, and definitely insulting.
In English this is sort of like saying you saw someone's trumpet performance vs show vs messing around vs screeching.