It can be a playful nickname for your rowdy sibling:
Sibling pretends to throw their
water bottle at you.
You flinch and respond with, "Whoa Scary."
-OR-
It can be what you call someone who just... really scares you:
You're standing in line at the grocery store and you catch a hater (or one of your many haters) in the next lane glaring a hole right through to your soul, making you feel really
uncomfortable. "I saw Scary at the store earlier, It was just glaring at me. Just made me feel really
uncomfortable, 'bout had to have a security guard escort me out."
Scary is their name, scary is who they are lol