The word "Arda" was apparently conceived as a cognate of English "Earth" and its equivalents in the other Germanic languages (such as Dutch "aarde", German "Erde"), although it ostensibly has no relation, being a Quenya word.
The word "Arda" was apparently conceived as a cognate of English "Earth" and its equivalents in the other Germanic languages, specifically the Afrikaans "Arde" which has equivalent pronunciation and meaning, and unsurprising in use given Tolkein's South African origins. However, within the lengendarium it ostensibly has no relation, being a Quenya word.