Silly

The polite way of telling someone they're a dumb ass without letting them know you're thinking they have recently acted or spoke without using even an nth of thought/brain activity.

Act: Your best friend insistently waving at your mom as you drive past her in the Walmart parking lot while skipping school.

BFF: Look! It's your mom. (Waving and yelling, "Hey Mrs. Connor")

You: Yeah but we are supposed to be in gym class right now, Silly!