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Like A Boss is a catchphrase often used in image macros or GIFs that feature a person completing an action with authority and finesse. Similar to Haters Gonna Hate, the characters in the macros have an air of superiority and do not care how others perceive them.
The phrase stems from The Lonely Island's single "Like A Boss," a parody of the Slim Thug song of the same name. Featuring Andy Samberg and Seth Rogen, the video premiered on Saturday Night Live as a digital short on April 4th, 2009. In the video, the Boss (Samberg) is asked to go over his daily activities during a performance review by Rogen's character. While it starts out with daily office tasks, the lyrics evolve into impossible activities.
Sample lyrics from The Lonely Island's Like a Boss 
Crash my car (like a boss)
Chop my balls off (like a boss)
Turn into a jet (like a boss)
Bomb the Russians (like a boss)
Crash into the sun (like a boss)
Now I'm dead (like a boss)
The day after the video aired on SNL, it was shared by the Huffington Post, Laughing Squid, Funny or Die and Digg, where it received 3645 upvotes. On June 5th, 2009 the site InstantBoss.org was created, which gave users a button that would play a single "Like a Boss" line.
On Memebase and Very Demovational, image macros have been more popular, with people adding the phrase to images of people completing actions easily or with expertise. As of September 2011, the song has been scrobbled on last.fm over 966,000 times by over 150,000 unique users.
Though "like a boss" was being searched for previous to the Lonely Island video, there is a clear spike that coincides with it in April 2009:
 Laughing Squid – Like A Boss by The Lonely Island, Featuring Seth Rogen