1680s, from French brilliant "sparkling, shining" present participle of briller "to shine" (16c.), from Italian brillare "sparkle, whirl," perhaps from Vulgar Latin *berillare "to shine like a beryl," from berillus "beryl, precious stone," from Latin beryllus (see beryl). In reference to diamonds (1680s) it means a flat-topped cut invented 17c. by Venetian cutter Vincenzo Peruzzi.
Excellent: Go outside for lunch? Brilliant!