Phonetic rendering of Spanish 'Don Juan' (DON-hwan):
唐 (chosen because the Tang dynasty connotes elegance and the 'don/táng' sound matches) +
璜 (a half-disc jade ornament, picked for the 'huan' sound). The Don Juan legend reaches Chinese audiences through Tirso de Molina's play, Mozart's opera 'Don Giovanni', and Byron's epic poem, making
唐璜 the standard literary shorthand for an unrepentant womanizer.