The heavy residue, produced as a result of the combustion of solid fuel, that collects at the base of a fire or combustion chamber; contrasted with "fly ash".
Origin
Early 20th century.
Definition of bottom ash in US English:
bottom ash
noun
The heavy residue, produced as a result of the combustion of solid fuel, that collects at the base of a fire or combustion chamber; contrasted with "fly ash".