A person who is a result of one having a thumb up their ass.
Having 'a thumb up your ass' is lazy but the ass thumb is so lazy that their not even doing anything.
Bob didn't do anything the whole week, what an assthumb.
(adj.) Not doing what you should be doing, be it at work or in free time. Pls note it can be 'thumb up my/your/his/hers/possessive pronoun's ass" as needed. I just put "your" in the entry title because it's the most common.
Manager bitching at employee at anywhere Wal Mart USA: Yo! Cletus! Stop sitting around with your thumb up your ass and get unpacking those crates.
Getting your thumb out of your ass means to stop wasting time doing nothing when you've got shit to do.
"Ugh, I hate English. It's so boring!"
"Yeah, well this is a Group Project, so you're not the only one one whose neck is on the line. So get your thumb out of your ass and help, you lazy fuck!"