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To find someone or something accidentally or without looking. Please don't be upset, we didn't mean to exclude you! When Chad and I went out, we just chanced upon the rest of the group, that's all! Were you snooping in my room? How else would you just chance upon my love letter?
chance upon (someone or something)
To happen upon someone or something accidentally or coincidentally. I chanced upon him in the library after work, and we got into a great conversation about Hemingway's works. I chanced upon a $20 bill on my way to school this morning!