The euthanasic term for "silly ass".
It's used for referring to someone who is extremely silly or just plain ridiculous.
It could also be used to poke fun and tease someone as it assumes that they are ignorant and immature/childish.
also equivalent to "silly goose"
Amy you're such a silly butt, I wish you'd just try new foods - it wont kill you!
Awww, what a little silly butt. He doesn't even know how many days are in a year!