From Middle English *erthfast, *erthfeste, from Old English eorþfæst (“earthfast, fixed in the earth, firm in the earth”), equivalent to earth + -fast.
"earth-fast." YourDictionary, n.d. Web. 10 June 2018. <http://www.yourdictionary.com/earth-fast>.
earth-fast. (n.d.). Retrieved June 10th, 2018, from http://www.yourdictionary.com/earth-fast