This how Red Necks say thank you in Spanish. It is a phonetic transliteration of "Grassy Ass." The Red Neck attempt at pronouncing Gracias.
Because the attempt at prouncing gracias is usually an honest effort at inclusivity Lawn Burro is way of gently referring to people who are kind hearted but don't quite get it.
Sometimes replaces De Nada.
non-Spanish Speaker, "Grassy Ass"
Spanish Speaker in reply, "Lawn Burro"