Definition of red flannel in US English:

red flannel

noun

  • 1Flannel dyed red and formerly used especially for underwear, nightwear, etc.; chiefly attributive designating a garment made from this cloth.

  • 2(In plural, informal) a garment made from red flannel.

Origin

Late 18th century; earliest use found in Thomas Bridges (?1710–?1775), playwright and novelist.