A spittoon is a bowl or other container that's made for spitting into. Your tobacco-chewing uncle might keep a spittoon on his porch.
Spittoons are fairly old fashioned — in the late 19th and early 20th centuries, it was common to find public spittoons, usually made of brass, in places like banks and hotels. You can really call anything that's used to hold saliva a spittoon, like a beer can or a coffee mug, although the word thankfully doesn't come up as frequently as it used to, since chewing tobacco is less common. In the late 1600s, it was called a spitting box.