Green’s Dictionary of Slang

1. to have a menstrual period.

1980

J. McClure Spike Island (1981) 148: One says, ‘I’m havin’ me period!’ Whereupon the other says, ‘I’m bloody floodin’ ’ere!’.

2. to have an erection, for the penis to ‘flood’ with blood.

1980

E. Folb Runnin’ Down Some Lines 237: flood Have an erection .