Not the best language, but it refers to its carbon footprint. The problem is that mining bitcoin takes computer processing energy, and if that energy is non-renewable, it's dirty. Bitcoin mining is dominated by China, which uses mainly hydroelectricity during the rainy season and coal the rest of the year.

Of course, you're right, it's not bitcoin itself that is dirty, it's the grid. But the incentive is there to use fossil fuels to mine bitcoin, as it's the most efficient in terms of cost/reward ratio, and time.

But to reject bitcoin on the basis of being "dirty", it's stupid because fiat and gold is so much dirtier, in terms of energy demands... gold needs digging out of the ground, so does the metal we make coins from, notes are now plastic, money needs security and transporting, banks need heating, and staff which have to travel to and from work, their executives travel the world, fiat is absolutely fucking filthy compared to bitcoin.