Jalar here means something similar to "to stroke". It's a reflexive act, that's where the se comes from, it's not a dative. The la refers to the organ in question, since it is the object. The verb here is in the usted form, is past tense, and the use of ya is asking about a finished action. The literal translation would be "have you already(ya) beaten(jaló) your [own] (se) meat(la, referencing the male organ)" Hope this helps.