on the down-low: (adjective phrase; American slang) secret, confidential

A: " What happened to you last night? We waited but you didn't show up."
B: "The police took my family's dog because I didn't have a license for it. I had to spend the whole night looking for a new dog that looked to the same as the old one."
A: "Really? That's crazy. "
B: "Look, could you keep this on the down-low? My parents loved the old dog. I don't want them to hear I bought a new one."