Dal tedesco Esche.

Che sembra avere radici simili ad alcune vecchie parole tedesche per la lancia, per la quale arma il legno è davvero ideale.

It’s a coincidence that ash the tree and ash the substance produced by burning wood look the same in modern English.

As recently as Old English, they were separate, with the tree being æsc and the substance being æsce.

Looking at the cognates, it seems that the tree may have been masculine and the substance femine. However, that was not the only difference, as we go further back, we see that the substance always had an a vowel in it (the Proto-Indo-European root is reconstructed as *ai-) while the tree originally had an o in it (PIE root *os-) that turned into an a in Germanic languages and a few other descendants of PIE.

Source: Online Etymology Dictionary