There’s one thing I always wanted to do before I quit–retire!

The line is a characteristically Groucho-style inversion: instead of “quit” leading naturally to “retire,” he treats retirement as an unfulfilled ambition—something one might never get around to before having to stop. The humor depends on the mock-serious framing (“one thing I always wanted to do”) applied to an ordinary life milestone, turning it into a punchline about work, aging, and the elusive promise of leisure. Implicitly, it pokes fun at the idea that people plan for a restful endpoint while life (or necessity) may force an exit before that ideal arrives.