There are several meanings of the word "brownie"

Table of contents
1 Mythology
2 Youth organization
3 Expressions

Mythology

Brownies were originally pixie-like creatures.

Youth organization

The name was adopted for the female version of Cub Scouts in Britain, i.e. the younger version of Girl Guides. Brownie packs are divided into subgroups of about six members with names such as Leprechauns, Pixies and so on. Their uniforms are traditionally brown and yellow. As the Scout Association now allows girls to join the Cubs (as in America), Brownies are reducing in number.

Expressions

"To be chucked out of the Brownies" is a punishment that has no fear or never takes place, a fake threat.

"Good Brownie points" means that it is in your interest to do something; an expression from US military slang for arse-licking.

The above expressions are still used in British English.