I think it's a Steven Wright
invention, but if it isn't, well, I'll attribute it to him anyways.
Meaning : The second mouse gets the cheese. The first mouse buys the farm. It speaks deeply to us pessimistic, paranoid types... "Well, Bob, it sure looks safe... why don't you try it first?"
This phrase is an effective response to the old, hoary standby The early bird gets the worm!. You uber-cynical types - when confronted in the early morn with a coworker chirping 'Wake uuuup! The early bird yadda yadda blech', thrust this response back in their tanned, well-rested mugs. You'll feel better. Slightly.