Map of England

Spotted dick

The generic British word for dessert is "pudding."

In the 19th century, the "g" was sometimes pronounced as a harder "k." Sometimes, the "n" got dropped. Sometimes that was shortened by slicing off the "pud."

In other words, small, incremental changes resulted in pudding->puddink->puddik->dick.

It's not meant to be dirty; it's just a Victorian synonym for "dessert."

Pepper a cake with currants or raisins, and you get "spots" in your pudding, hence: spotted dick.