I found it in a slang dictionary (not that I heard of it before either) "Titiri" is used to express disdain for someone missing the obviousness of something. It is clearly an exclamation. -Did you know that he is an absolute jerk? - Titiri! I should have noticed that. It means : " Oh my or shame ,it is so obvious ,how didn't I notice it.

I've never heard of it, where did you hear it being said and in what context?

Not every native English speaker knows all the English slang - in many cases, it's been made up for an isolated situation. I certainly have never used "titiri", but it's apparently similar to "pish-tush".