Green’s Dictionary of Slang

bright adj.²

healthy.

1959

C. Lee diary 17 May in Eight Bells and Top Masts (2001) 218: I’m not feeling too bright. I keep getting a pain in my belly .

In phrases

on the bright side (of)

(US) younger than (a specified age).

1857		Christian Miscellany vols. 5-6 12: [headline] ‘the right side of fifty’ and ‘the bright side of seventy .
1912		Talmadge & Talmadge T. De Witt Talmage as I Knew Him 251: [John Greenleaf] Whittier was on the bright side of eighty then.
1916		Chemist and Druggist n.d. 38/2: Mr Baynard was on the bright side of fifty , and passed the Minor examination in July 1892.
1984		A. Sample (con. mid-1960s-early 1970s) Racehoss 278: Big Devil was on the bright side of sixty.