This was posted on the groaners listserv. The author is not known.
Late one afternoon Tarzan comes swinging through the trees into his treehouse and says “Jane, quickly a martini. Make it a double.” He drains it in a single swallow and says “Another double!” This he also drains immediately.
Jane asks, “What’s the trouble?”
Tarzan shakes his head and says, “It’s a jungle out there!”