spring something on someone
1. to surprise someone with something. I hate to spring this on you at the last moment, but I will need some money to travel on. Please don't spring any other demands on me.
2. to pull a trick on someone. Let me tell you about the trick I sprang on Sally. What are you going to spring on her this time?
spring (up)on (someone, something, or an animal)
to jump on someone, something, or an animal; to pounce on someone, something, or an animal. (Upon is formal and less commonly used than on.) The lion sprang upon him and knocked him down. The cat sprang on the mouse and captured it.