Bad Advisor is a meme established by the Dominion community after the release of Guilds. It involves text overlaid on the card art for Advisor giving bad (usually humorous) advice, usually Dominion-related.
The meme springs from the confluence of the mechanic of Advisor and its art. When played, Advisor reveals three cards from the player's deck, and the opponent to their left chooses one of them for the player to discard. This usually ends up being the "best" card of the three, leading to the idea that the Advisor gives bad advice. The card art, depicting a lackey grinning while whispering into a king's ear, furthers this impression.