doof
A combination of the words "dork" and "goof", usually used in a joking manner. However, when someone does something really stupid it can be used to point out their stupidity.
Friend: I saw you in the Bio room yesterday!
Me: I wasn't at school yesterday, ya doof!

Friend: We walked past the marshmallows twice while we were looking for them!
Me: Wow, ya big doof!