I've tried to find the origins for biot before and got nowhere. But when I looked up Flue in the Concise Oxford Dictionary on my PC it said a pipe to carry away smoke but then under fluff I found -
fluff // n. & v.
n.
1 soft, light, feathery material coming off blankets etc.
2 soft fur or feathers.
3 slang a a mistake in delivering theatrical lines, in playing music, etc. b a mistake in playing a game.
4 something insubstantial or trifling, esp. sentimental writing.
v.
1 tr. & intr. (often foll. by up) shake into or become a soft mass.
2 tr. & intr. colloq. make a mistake in (a theatrical part, a game, playing music, a speech, etc.); blunder (fluffed his opening line).
3 tr. make into fluff.
4 tr. put a soft surface on (the flesh side of leather).
bit of fluff slang offens. a woman regarded as an object of sexual desire.
[probably dialect alteration of flue ?fluff?]

So it looks like flue in this context is a dialect word.

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Donald/Scotland