What is a fast buck?

What is a fast buck and where does it comes from?

It's a dollar, a "buck," made by illegal or unethical means, not one earned by the sweat of honest labor. (I was going to say it's a male deer and it comes out of the woods, but I restrained myself.)

Usu acquired quickly

Easy money. Usually through something questionable or downright illegal. The opposite of earned an honest dollar.