Literally, βapple of [the] earthβ. The word pomme used to mean "fruit" in Old French. The French construction originated, as calques, Dutch aardappel, Icelandic jarΓ°epli, Persian Ψ³ΫΨ¨βΨ²Ω ΫΩΫ (sib-zamini), Modern Hebrew ΧͺΧ€ΧΧ ΧΧΧΧ (tapΓΊakh adamΓ‘), the rare English earthapple, German Erdapfel, etc.
In fact, apple was a catch all term for fruits in many languages from time to time, hence pineapple (originally meaning pinecone, later used for the exotic fruit because of similarity) or German Apfelsine (orange, literally apple from China), ...
That's actually not true, 'ground apple' is a common name for different sorts of tubers in a number of different languages, going back to the latin 'malum terrae'.
That is news to me. Never thought to dig too deeply into my French studies in middle and high school (two decades ago), and so "apple of the earth" was just appropriate. Like, yeah, why wouldn't it be apple of the earth?