The term "ass" is a vulgar slang word for donkey. It can also be used as an insulting way to refer to someone who is stupid or foolish, similar to calling them a "jackass."
Usage examples
The donkey had a stubborn and disobedient temperament, kicking and lashing its ass around.
Alex made an inappropriate joke at the party, causing everyone to raise their eyebrows and give him a thorough chewing out for being a smart ass.
The detective examined the crime scene, looking for any fingerprints or other evidence that could help crack the case wide open and catch the culprit's sorry ass.
Katie was tired of her coworker's laziness and lack of effort, so she reported his lazy ass to their boss.
The soccer player took a powerful shot, hitting the ball with incredible force and scoring a magnificent goal with a brilliant kick of his ass.