Green’s Dictionary of Slang

flannel n.2

[? 19C tradesmen’s jargon flannel, the ornate, scroll-ridden letterheads with which tradesmen garlanded the invoices they sent to their aristocratic clients. There is no proof, however, that this is linked to the 20C use, albeit a similar one]

rubbish, nonsense, albeit plausible rubbish.

[UK]A.N. Lyons Hookey 132: An’ the missis will say: [...] In our day, servants knew their places – which is red flannel, of course.
[UK]Daily Express 11 Oct. 3/4: One day his sister died sudden. Up he comes to ask for fourteen days’ leave [...] ‘to mourn over the body [...] according to the Jewish faith’ [...] The padre wired to a rabbi [...] it was all flannel [...] just flannel from beginning to end.
[UK]D. Bolster Roll On My Twelve 30: All a lot o’ flannel an’ Do this and Don’t do that! Pah!
[UK]J. Curtis Look Long Upon a Monkey 198: She went for Tony’s old flannel.
[UK]A. Buckeridge Speaking of Jennings (1989) 23: You know, falling leaves and autumn tints and all that sort of flannel.
[UK]‘John le Carré’ Smiley’s People 49: Wise, my Aunt Fanny. Bunch of left-wing flannel merchants.
[UK]S. Armitage ‘Ivory’ in Zoom 74: No jackassery, or flannel, / or galumphing.
[UK]S. Maconie Pies and Prejudice (2008) xiv: I hope this book is a love letter [...] but not just flannel and boasting.

In compounds

flannelhead (n.)

a fool.

[US]J. Archibald ‘State Penmanship’ Popular Detective Jan. 🌐 ‘But Slapnicka said it was forged, flannelhead,’ Mr. Gerke yelped.