1. Doing something very dangerous that you may/may not get away with. Sometimes the dragon wins.
2. The origin of this phrase came out of the USA Manhattan project to develop the atomic bomb. Nuclear scientists would induce ever increasing levels of criticality - close to the point of a nuclear chain reaction - a very dangerous and lethal undertaking. The term "tickling the tail of a sleeping dragon" is accredited to the Nobel laureate Physicist Richard Feynman who was involved with the Manhattan project.
2. Having random one-night-stand sex without a condom is tickling the dragons tail.
3. Secretly having sex with your wife's sister is tickling the dragons tail.
4. Using a screwdriver to separate beryllium and plutonium is tickling the dragons tail.