One way to hold this centre: something is tied to the ground and cannot get up. Bound here is the bound of a parcel tied with string. Picture a plane on a runway in a storm, a rocket still on its stand, a bird that has never left the grass. In every case the thing could be expected to rise, and cannot. That held-down feeling is the whole of this centre, and it works just as well on a mind as on a machine.
From the word’s history
Wiktionary analyses earthbound as earth + -bound, with earth traced back through Middle English erthe to Old English eorþe. The packet does not say which -bound this is, but both of the word's attested uses show that English has a tied-down -bound and a heading-for -bound.
Wiktionary 把 earthbound 分析成 earth ＋ -bound，earth 一路從古英文 eorþe、中古英文 erthe 傳下來。資料沒有說明是哪一個 -bound，不過這個字被記錄下來的兩種用法本身就顯示：英文有「被綁住」的 -bound，也有「往…去」的 -bound。
Wiktionary, "earthbound"
登入就可以看每一支怎麼連回核心畫面。