The word "灰烬" is formed by combining the character "灰", which signifies ash and originates from a pictographic representation of burnt residue, with "烬", which denotes cinder or burnt remains and is composed of the fire radical "火" to link it to combustion and the component "尽" that provides phonetic cues and semantic notions of exhaustion. Together, they create a compound term precisely referring to the solid residue left after a substance is fully burned.