Commodities guru Dennis Gartman is not backing off his dramatically bearish stance on bitcoin.

"This is a market ... for criminals, this is a market for millennials," Gartman told CNBC's "Fast Money" on Monday. "This is a market for pure punters, but there is no value here whatsoever."

Even the cryptocurrency's tendency to bounce back in a big way from steep declines is not swaying Gartman.

"How can you buy or sell a painting using bitcoin, when the change in volatility is 20-30-40 percent in the course of a week? It's nonsense," he said. "I shall not trade it."

Bitcoin fell about 20 percent over the weekend, but regained much of its losses. Historically, the cryptocurrency has gained about 28 percent in the weeks following a drop of 20 percent or more.