"Adult swim" is a reference to public swimming pools, which would set aside the first 15 minutes of each hour for lap swimming. Every child dreaded the words because their play-time had to stop every hour when they had to leave the pool to make room for the adults. It pretty much means that the cartoons aren't for children.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.