It basically means that we shouldn't question or analyse a situation but accept the decision. It's usually said when someone above us (eg. management, owner, etc) has made a decision which seems dubious or just strange. Instead of trying to find a reason, we simply say "our is not to reason why" and leave it at that. Ours = our part in this. So "ours is not to reason why" can be expanded to: "our part in this situation is not to search for a reason as to why it is so".

Nono nozturk! Don't leave Italki for Google. Please ask here. It helps others to learn nice expressions. As for me, I've never run into most of them. By the way, where do you get them?:)

There's this wonderful thing called Google, which will answer all your questions about idioms and save you the trouble of posting them here. I'll get you started: http://www.idiomdictionary.com/definition/theirs-ours-yours-etc-not-to-reason-why.html http://www.idiomsite.com/ http://www.learnenglishfeelgood.com/americanidioms/index.html