The "a" in "earth" doesn't stand for anything. It's simply a part of the word that has evolved over time from its Old English and Proto-Germanic roots.

Here's a breakdown of its history:

* Old English: "eorðe"

* Proto-Germanic: "erþō"

The "a" in "earth" comes from the vowel shift that happened in the development of English from Proto-Germanic. While the "a" doesn't have a specific meaning, it's an important part of the word's sound and structure.