Originally, to trick someone into watching the extremely catchy "Never Gonna Give You Up" music video by the genius Rick Astley with a fake link (usually with the promise of an exclusive video).
Now used in a more general sense, for any sort of deception or trick.
"I asked for the newest Halo trailer and all I got was the Rickroll video."
"I really hope that J.K. Rowling hasn't rick rolled us with all this Dumbledore business. I would be so disappointed."
"I just rick rolled that guy into singing Evanescence at the top of his lungs in the middle of a Walmart by promising him a reward of $20,000."
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