“Caved(Excavated)!!!!” was an Engrish term in the Touhou Project community that borrowed the were-hakutaku, Keine Kamishirasawa, who would use her hakutaku form's horns into somebody's anus for mostly comedic purposes. Nowadays, it's mostly an ancient meme that is rarely talked about.

Of course, the joke of 'ramming things into someone's anus' is obvious, but there was also the added context that 'horu' (掘る, lit. “dig” or “excavate”) was Japanese slang for gay anal sex. With the primitive state of machine translations in the 2000s, Nifty translated 'horareta!!!!' (掘られた!!!!) as 'caved!!!!', so the Japanese community was stuck with this mistranslation until the joke had finally ran its course.

Now, the presumed logic behind 'caved' is that one definition of 'cave' is 'to hollow out', but the problem is that this definition is obscure, so most native speakers would default to the 'collapse' (cave-in) or 'surrender' definitions and be confused. In the end, none of this even matters if you understand that people will sometimes use 'gratuitous English' to be cool, cute, or exotic.

• Personally, I'd translate 'horaneta' (掘られた) as 'excavated', especially when 'kussakuki' (掘削機) is normally be translated as 'excavator' anyways. There's also 'dug', but it isn't as versatile.
• At some point, 'Caved!!!!' used to be the default message that would be shown when a reply would be deleted in Touhou Project boards on Shitaraba, mimicking the 'a bone' (あぼーん) on 2channel.

• Caved!!!! on Pixiv Encyclopedia
• Caved!!!! on Niconico Encyclopedia

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