The headword phrase literally means "you are clashing/clattering well." It appears in Plato, Euthydemus 293D (web address 1 below) and the entry derives from a gloss on the passage. Similar entries in Photius and Hesychius, and in Timaeus, Lexicon Platonicum.
The idiom has been taken (in LSJ and elsewhere) to derive from a noisy sort of game, πλαταγών or πλαταγή ; cf. Aristophanes fr. 116 Kock, and see the LSJ entries on both of these in web addresses 2 and 3. However, note also alpha 4121, pi 1697 and pi 1698 on a similarly-named percussion instrument.