unbeknownst to me

Unbeknownst to me is an idiom which means something is without the knowledge of a specified person or group of people. Typically someone will say something is unbeknownst to me when they were not previously aware of someone who was just said to them, and it is a very surprising thing because they had no prior knowledge of it. This is not a phrase commonly used in everyday conversation, but you might hear it spoken occasionally.