Green’s Dictionary of Slang

wonderful! excellent!

1983		J. Sullivan ‘Healthy Competition’ Only Fools and Horses [TV script] Triffic, brill Rodders, had a blinding week.
1996		K. Lette Mad Cows 269: The New Zealand niece’s plain impudent face lit up. ‘Brill.’.
2000		Guardian 17 Mar. 24: Brill! Totally good hair day!