Turns out, Grantham was only getting started.

Doubling down on his apocalyptic message, the one-time value investing guru told CNBC that the US stock market is in a unprecedented bubble and investing in it is "simply playing with fire."

"I have been completely amazed," the veteran bearish investor said in an interview Wednesday on CNBC. "It is a rally without precedent - the fastest in this time ever and the only one in the history books that takes place against a background of undeniable economic problems."

His advice to an entire generation of young daytraders jumping into the market now should sell U.S. stocks, buy emerging market equities and "throw the key away" for a few years, he said, adding "this is becoming the fourth real McCoy bubble of my career."