*gno-

The core idea of this root was

knowing

It’s the origin of can and to know and a big branch of Latin and Greek words that are all about knowledge in some way.
Here’s a little (incomplete) overview:

• can, could (“knowledge, skill”)
• to know, knowledge, acknowledge (“to know”)
• cognition (“get to know”)
• agnostic (“not knowing”)
• ignore, ignorant (“not knowing”)
• incognito (“unknown”)
• connoisseur (“the knower”)
• diagnose (“know about someone else”)
• recognize (“know again”)
• physiognomy (“knowledge of the physique”)
• notice, notify
• noble (“the known ones”)