flue

Etymology 1

From Middle English flue, flewe (“mouthpiece of a hunting horn”), of uncertain origin. Perhaps a back-formation from Middle English *flews (mistaken as a plural), from Old English flēwsa (“a flow, flowing, flux”). Alternatively, perhaps an alteration of Middle English floute, fleute, flote (“a pipe”), see English flute. Compare also Middle Dutch vloegh (“groove, channel, flute of a fluted column”).

noun

1. A pipe or duct that carries gaseous combustion products away from the point of combustion (such as a furnace).
   It has frequently been a subject of inquiry, whether the ancients were acquainted with chimneys, or open fire-places. In the houses discovered at Herculaneum and Pompeii, there are no chimneys; they all appear to have been warmed by furnaces and flues. 1815, Robertson Buchanan, A Treatise on the Economy of Fuel, and Management of Heat, Especially as it Relates to Heating and Drying by Means of Steam, Appendix, p. 307.
   Besides the usual run of machines, planers, millers, automatics, centre lathes, cranes, etc., there were several power stations, the rolling mills for strip material and for 60 ft. rails, and all the steel furnaces with their complicated systems of flues. If variety is the spice of life, then there was plenty here. 1944 November and December, A Former Pupil, “Some Memories of Crewe Works—II”, in Railway Magazine, page 341
2. An enclosed passageway in which to direct air or other gaseous current along.
3. (obsolete, countable and uncountable) A woolly or downy substance; down, nap; a piece of this.
4. In an organ flue pipe, the opening between the lower lip and the languet.

Etymology 2

adj

1. (UK, dialect) Alternative form of flew (“shallow, flat”)