The fall-back plans you make in case your life goes to complete shit, with no hopes of getting back on track. A Grenade Plan means blowing up your entire life and starting fresh. Grenade Plans are meant for worst-case scenarios only, and are usually extreme and nearly impossible.
Well, this week I got fired, my girlfriend dumped me, and I found out that my landlord is evicting me. My car was stolen, and I have no insurance. My family is all dead, I have no friends, and my pets ran away. My savings were lost in the stock market, and I have this odd lump on my neck. It can't get any worse, but it probably won't get much better, either. It's time for the Grenade Plan: Sell everything I have left, and bet it on a spin at the roulette table. If I win, I move to Monaco. If I lose, I'll become a busker in San Francisco.