(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.