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“You Are A Pirate” is a song from the children’s TV series LazyTown, episode “Rottenbeard” (aired October 4th, 2004). This song became very famous in the Internet mostly because of numerous user created parodies/recreations.
A number of memetic materials involving Stephanie and other LazyTown characters have their roots in YTMND, and “You Are a Pirate” is a prime example.
The “You Are A Pirate” YTMND first appeared in mid 2006, and is thought to be its first memetic instance.
The original video on YouTube:
“You Are a Pirate” 
Music by Mani Svavarsson
Lyrics by Ken Pontac
Performed by Stefán Karl Stefánsson and Julianna Rose Mauriello
The video on YouTube had over 5 million views (orginal youtube video removed). Its popularity might have its source in the combination of the following: Weirdness (LazyTown) + Coolness (the song, although the Stephanie part is often considered lame and left out or made fun of in user-created versions).
Many users decided to delete the weirdness factor by using the song with their own animations, especially the “LOL, Limewire” Flash animation, the most popular variation of this meme by far.
Search interest shows the trend began in mid-2006, with some renewed interest in early 2010 due to the “LOL, Limewire” variation.