on someone's ass

• on someone's butt

on someone's ass

2. (Canada, US, colloquial, vulgar) Rapidly chasing or pursuing someone.

   Help! The cops are on my ass!

   • 1987, Samantha Harte, Vanity Blade, Toronto, Ont.; New York, N.Y.: PaperJacks Ltd., →ISBN, page 243:

     Then they saw him, his legs pumping, his face a mask of determination. "Go!" he shouted in a high voice. "They're on my ass!"

4. (Canada, US, colloquial, vulgar) Tailgating someone.

   Why is this guy on my ass? I guess doing ten over the limit isn't enough speeding for this moron.

6. (Canada, US, colloquial, vulgar) Persistently urging someone on and putting pressure on them.

   Coordinate term: up someone's ass

   My mom's always on my ass about keeping my grades up.

   • 2011, Harry Carson, Captain for Life: My Story as a Hall of Fame Linebacker, New York, N.Y.: St. Martin's Press, →ISBN, page 44:

     They would see to it that I graduated, not by pulling strings as might have been done at other schools, but by staying on my ass just as a parent would to help me off the field as well as on the field.

   • 2012, Mark Anthony Rolo, My Mother Is Now Earth, St. Paul, M.N.: Borealis Books, →ISBN, page 121:

     I slept on his couch for a few days, then he started getting on my ass about looking for work. I said, 'For Christ's sake, Frankie, I just graduated. Can I just enjoy the moment?'

• The term is usually considered disrespectful and contemptuous in formal settings, but is often used positively in informal settings.

• “be on someone's ass”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “on one’s ass”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.