to the moon
To The Moon historically refers to a series of images that came out between 2010 and 2013 joking about the My Little Pony: Friendship is Magic series. In the series a major character, Princess Celestia who is basically the goddess of the sun, banishes her younger sister Princess Luna, who is basically goddess of the moon to the actual moon, only for her sister to return 1000 years later as a much more sinister and powerful Nightmare Moon.
The images joke that Princess Celestia made little to no efforts to solve the problems that lead to her sister's exile or prepare for her potential return and that she seems like the type that would get rid of all her problems by sending them to the moon are find any excuse to send things to the moon similar to the Queen of Hearts from Alice in Wonderland.
More recently, the tag has come to refer to any situation where a character is suddenly and unwillingly sent to the moon.