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)]

grog, punch or gin-twist, with a dash of beer.

implied in hot flannel
[UK]G. Parker Life’s Painter 152: Hot. A mixed kind of liquor, of beer and gin, with egg, sugar and nutmeg, drank mostly in night-houses, but when drank in a morning, it is called flannel.
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.
[US]Donaldsville Chief (LA) 7 Sept. 1/2: The names of the drinks most in vogue [...] yard of flannel, locomotive, corpse reviver [...].
[UK]Hotten Sl. Dict.
[UK]Portsmouth Eve. News 8 Nov. 3/5: American Drinks [...] gum tickler [...] a ‘yard of flannel’, washed down with an ‘eye opener’.

SE in slang uses

In compounds

hot flannel (n.) (also warm flannel)

heated gin and beer with nutmeg, sugar and spices; also attrib.

H. Lemoine ‘Clever Fellow’ in Wit’s Mag. 155/2: My mott oft’ tips the knowing dive / When sea-crabs gang the stroll; / Unless she did how could we thrive, / And in warm flannel roll?
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 190: Warm flannel — spirits, mixed; hot, perhaps.
[UK] ‘Her Muns with a Grin’ in Swell!!! or, Slap-Up Chaunter 50: No sweeter her buss — is a hot flannel nurse.
[UK]Flash Dict. in Sinks of London Laid Open.
[UK]A. Mayhew Paved with Gold 269: Master Billy, who was commissariat-general company, ordered a jug of what he termed ‘hot flannel’ for three – a mixture of gin, beer, and eggs – which he declared wrapped round a fellow like wool, and made him sleep like opium.
[UK]Hotten Sl. Dict.
[UK]Eve. Teleg. (Dundee) 1 Sept. 3/6: The language of the London East -end pub [...] ‘Hot flannel’ — Gin and hot water.