Green’s Dictionary of Slang

flannel n.1

[SE flannel, a form of woollen cloth; the drink ‘keeps one warm’; ‘a play on the old name “lambswool.”’ (Hotten, 1874)]

[early–mid-19C] grog, punch or gin-twist, with a dash of beer.

SE in slang uses

In compounds

flannel-jacket (n.) (also flannel back)

[late 19C] a navvy, who wears such a garment.

hot flannel (n.) (also warm flannel)

[18C–19C] heated gin and beer with nutmeg, sugar and spices; also attrib.