From《THE SHAWSHANK REDEMPTION》：
Like he did, You're gonna open it or stand there with your thumb up your butt?
what does " your thumb up your butt" mean?
Literally, it means "with your thumb in your anus".
The meaning is "not doing anything useful". It's just an intensifier. Are you going to open it, or just stand there ... <insert insult>? ... looking stupid.
... like a stunned mullet. ... and wait for Christmas.