Justin Bieber is an oblivious, self-important little twit who goes out of his way to make the working man's life miserable -- just watch this video of the singer pissing into a restaurant mop bucket ... and laughing like he's the king of the world.

The clip was shot in NYC earlier this year -- we're told Bieber and his idiotic friends were leaving some nightclub, exiting through a restaurant kitchen, and Bieber decided he needed to take a leak.

Read more: [link to www.tmz.com]