It used to be that washing was a way to make something clean, and as such it was a positive term. Nowadays washing is more of a negative term, since we have white-washing, green-washing, and even farm-washing is a thing now!
That last one is the nasty habit of food stores to act as if their food comes from local farms, when in fact this is not the case...
Green-washing is saying you are doing something more the environment than you are actually doing. Now personally I am not a real believer in the whole environmental threat, it looks more like fear mongering if it is just a bunch of billionaires flying private jets that are telling us we can't drive a simple gas car, and we all need to buy solar panels. That is just about keeping the consumption going.