the princess is in another castle

1) Your girlfriend is cheating on you.
2) She ain't here.
3) You tried really hard, but you failed.
4) You just got pwned by either Princess Peach or Princess Daisy.

1) The event in which you expect a female, only to get something else (like a dude) instead, especially in the case of dating a transvestite or someone similar. The phrase originates from Super Mario Bros., where instead of getting the princess at the end of all but the last castle, you end up rescuing a Toad instead.

2) The event in which a female does not show up at the appointed place and/or time.

Similar to these aren't the droids you're looking for, Your Princess is in Another Castle is a phrase used to tell someone they're looking in the wrong place or otherwise barking up the wrong tree, often used sarcastically.

The phrase refers to a cutscene in the original Super Mario Bros. game which became a well-known gaming meme, seen at the end of every castle save for the final castle in World-8: A Toad would be waiting for you in the room immediately after the boss fight with Bowser and say, "Thank you, Mario! But our Princess is in another castle." This became increasingly frustrating, since, this being a classic Nintendo hard game, the difficulty spiked dramatically between each castle, and reaching the next one, let alone beating it, always required an even more ridiculous amount of skill.

An appropriately mocking response to single men when they complain about not being able to find their ideal mate. Originated from the game Super Mario Bros. when at the end of every world save the last one Mario would be greeted by a mushroom man who would inform him "Thank you Mario! But our princess is in another castle!" And when he does finally find the Princess she just sends him out to do it all over again with more difficult challenges. Represents the futility of the search for a mate in a humorous way to which the children of the 80's can easily relate.

What you get for saving toad.

What toad says to you after you that fat turtle's ass.