flue 1 
(fl)
(fl)n.
1. A pipe, tube, or channel for conveying hot air, gas, steam, or smoke, as from a furnace or fireplace to a chimney.
2. Music
a. An organ pipe sounded by means of a current of air striking a lip in the side of the pipe and causing the air within to vibrate. Also called labial.
b. The lipped opening in such a pipe.
[Origin unknown.]