Is thank you from the bottom of my an idiom or a simile?

The phrase "thank you from the bottom of my ____" is an idiom, not a simile. A simile is a figure of speech that makes a direct comparison between two unlike things, using the words "like" or "as." For example, "She is as happy as a clam." An idiom is a phrase or expression that has a figurative meaning that is not easily deducible from the literal meaning of the phrase. For example, "to spill the beans" means to reveal a secret. In the case of "thank you from the bottom of my ____," the idiom is completed with a body part, such as "heart" or "soul," to express sincere gratitude.