Flue dirt is a type of SOOT that accumulates in chimneys and flues. It is composed of a variety of materials, including ash, tar, and other combustion byproducts. Flue dirt can be hazardous to health if it is not regularly removed from the chimney or flue. The term 'flue dirt' is derived from the four-letter word 'SOOT, ' which is a black or dark gray powdery or flaky substance made up of particles of carbon.