“Outside of a dog, a book is man's best…” – Groucho Marx

“Outside of a dog, a book is man's best friend. Inside of a dog it's too dark to read.”

– Often attributed to Groucho Marx

What does it mean?

A joke built on two meanings of "outside of". First it means "apart from" (a book is the best friend after a dog), then it is taken literally: you can't read inside a dog.

Who said it?

Groucho Marx: American comedian and actor (1890–1977).

These exact words are widely credited to Groucho Marx, but they don’t appear in their known work. They may be a later paraphrase or a misattribution.