I thought it was a euphemism for a dick which the author used in another story.

Edit: yup, it’s dick

In 1979, Dahl decided to revisit snozzberries in his adult novel My Uncle Oswald.

The term “snozzberry” comes up when Yasmin Howcomely recounts her experience with George Bernard Shaw:

“How did you manage to roll the old rubbery thing on him?”

“There’s only one way when they get violent,” Yasmin said. “I grabbed hold of his snozzberry and hung onto it like grim death and gave it a twist or two to make him hold still.”