[Vocabulary] I can't think of a word

[Jeffm03]

It's a word that is used when someone says "he's in his own world ", except someone would say "he's in (*word*) earth". Makes it sound like that person is really in their own world and they are sure of it but i can't remember people rarely use this word. I've tried using a thesaurus and looking up words like euphoric and others but no list seems to have this word.

 

[emsr2d2]

It's a word that is used when someone says "he's in his own world ", except someone would say "he's in (*word*) earth". Makes it sound like that person is really in their own world and they are sure of it but i can't remember people rarely use this word. I've tried using a thesaurus and looking up words like euphoric and others but no list seems to have this word.

I can't think of a BrE phrase involving "He's in ***** earth" which means someone being in their own world.