Green’s Dictionary of Slang

red rag n.

1. [late 17C–late 19C] the tongue; thus too much red rag, speaking too long and too loud, give the red rag a holiday, be quiet, stop talking, wag the red rag, to talk to excess [its colour and its ‘flapping’].

2. [late 19C+] a menstrual cloth or sanitary towel.

In derivatives

red raggism (n.)

unrestrained, critical speech.

In phrases

flash the red rag (v.)

[early 19C–1900s] to menstruate.

mount the red rag (v.)

[19C] to blush.