on (one's) ass

1. rude slang Driving too close to the rear of one's car. Primarily heard in US. Why is this guy on my ass when I'm already going over the speed limit? You don't need to be on their ass like that—if they brake all of a sudden, you'll run into them!

2. rude slang Constantly criticizing or putting pressure on one, especially in an irritating or domineering manner. Primarily heard in US. My mother has been on my ass this entire visit, questioning every aspect of my life from my marital status to the improper folding of my linens. Sometimes you've got to be on your team's ass if you want them to get things done on time.