What does "¡Ay Mijo!" mean?

So according to Urban Dictionary “mijo” is defined as follows: Conjoined spanish slang of affection. Mi + hijo, “my son.” Can be said to any man or boy, usually by an older person. Can also mean “my dear” “sweetie” or “hun.” Also directed to guys by thier wives, girlfriends, or any female freind and/or peer.

Not sure I agree with that last portion about the usage by a significant other, but the rest of it is pretty accurate. This weekend my niece asked: “grandma always says ‘ay mijo’ when she talks, why does she say that?” It’s something she’s always said so her question kind of caught me by surprise.

Did she honestly not know what that meant? Just in case here are a couple of examples just to make the definition a bit more clear.

¡Ay mijo! Estás muy flaco… 

¡Ay mijo! ¿por qué no comes? 

¡Ay mijo! ¡no te dejes! 

¡Ay mijo! Te quiero mucho… 

¡Ay mijo! No te preocupes…

¡Ay mijo! Que tonterias dices…  

¡Ay mijo! I love you like a fat kid loves cake.