The headword phrase literally means "you are clashing/clattering very well." It appears in Plato, Euthydemus 293D (web address 1 below), and the entry derives from a gloss on the passage. Same or similar entries in other lexica, including Timaeus' Platonic Lexicon.
The idiom has been taken (in LSJ and elsewhere) to derive from a noisy sort of game, platagh/ or platagw/n (sic); see the LSJ entries on both of these in web address 2 and web address 3. However, note also alpha 4121, pi 1697 and pi 1698 on a similarly-named percussion instrument.