In bocca al lupo

which means, literally, 'into the mouth of a wolf'. It's an idiom which means 'good luck' (don't ask me why, I've already asked my Italian colleagues and it seems that it could be related to the story of 'The little Red Riding Hood' but no one knows exactly why). And to this, one should not say 'Grazie' (thank you) in reply but 'Crepi!' which means 'May it die!'.
And my Italian colleagues went on to tell me that another Italian idiom for good luck is 'In culo alla balena!' (Into the ass of a whale!), and the response to which is 'Speriamo che non caghi!' (Hope it doesn't defecate!) hahaha... Funny huh? They added that this is a more 'vulgar' idiom and I shouldn't use it, for example, when talking to my boss.
A quick search online led me to a page with Italian proverbs on Wikiquote and there was another variant to say good luck: 'In groppa al riccio!' (On an hedgehog's back!), and the appropriate response is 'Con le mutande di ghisa! (Wearing underpants made of cast iron!)
Learning a new language is tough (a lot of brain cells have to be summoned in the process which proves more difficult the older one gets), but is very interesting seeing how it brings me that bit closer to the culture, and hopefully, when I can understand more and speak, its people. :)