What is "stumble upon" means?
I search "stumble" on the dictionary.
And get a sentence like "He stumbled on the staircase and hurt his leg."
OK, I understand that.

But I also get a sentence "I stumbled upon the rare book in a second-hand bookstore."
To me, "stumble" and "stumble upon" have totally different meanings.
I want to know the linkage between them.

Question

What is "stumble upon" means?
I search "stumble" on the dictionary.
And get a sentence like "He stumbled on the staircase and hurt his leg."
OK, I understand that.

But I also get a sentence "I stumbled upon the rare book in a second-hand bookstore."
To me, "stumble" and "stumble upon" have totally different meanings.
I want to know the linkage between them.

Answer

It means to find something you weren't looking for.
EXAMPLE:
"I stumbled upon this old picture while I was cleaning my room. Why didn't you tell me this existed?"

Answer

"Stumble upon" means that you were not looking for something but found it out of pure coincidence or luck. If someone says "I stumbled upon a great restaurant!", that means that there were not looking for this restaurant and were not expecting to find it, but found it anyways.

Answer

In this sentence it means that you found this rare book even though you weren't actually looking for it and it was rather unlikely that you would find something this rare in this place. We often say we stumbled upon a good bargain in a store. It is generally a positive outcome of some type that we were not expecting.