Green’s Dictionary of Slang

(Aus.) a drinker who cannot hold their liquor, a ‘lightweight’.

2007		Urban Dict. 15 May 🌐 Cadbury's girl Person who becomes drunk after only a glass and a half. From the famous recipe for Cadbury's chocolate which has 'a glass and a half of milk'.
2021		Betoota-isms 259: ‘Never get stuck taking care of Mitch on a night out, there’s a reason they call that fella Cadbury’.