An opened but unfinished can of beer left out until it's warm. A full royball is a beer that has been opened and then never touched again. Royballs are considered a huge waste of beer and an act that brands you as a pussy. Understandably, the last sip is considered backwash, sauce, foam, froth, or some other undesirable description and it is o.k. to leave a tiny bit in the can equivalent to a sip. Anything more than a sip left in the can is considered a royball. An addendum to the "man-code" should be: thou shall not leave royballs.
I found like 30 royballs while helping clean up after Farney's party. One of them spilt on my shirt and there was 3 cigarette butts in it, I almost puked.