Actually the first sentance I was ever taught. I often say it wrong because there is a gutteral "aiyn" [sound non-existant in English] softly eununciated in the middle of the word. It is "moo" [like a cow] "aiyn" [like the arabic] "Yaa-nee" and then "nuh-duh-hak" [Arabic word for laugh].
It means, "should we laugh now?" Kind of the Omani equivelant of sarcasm, and is a very old Omani word which makes people scrunch up their faces at me, and Omani girls giggle, "where did you learn THAT????!!!!" Yes, thank you K for making people laugh at me, always.;)
Apparently Princess and BL were to say it whenever any of their old shebab were to make a joke or to say something silly that the guys thought was funny.
I honestly don't know if it is rude or not [cuz I never know with K] , but seems to make people smile, so I guess from a Westerner using it, it is not.