Green’s Dictionary of Slang

(US) menstruation; thus grandma’s coming.

1953		Randolph & Wilson Down in the Holler 106: Old grandmaw’s coming, which refers to menstruation.
1966		Current Sl. I:3 3/2: Grandma george, n. Menstruation, v. To be menstruating.
1967–8		Baker et al. CUSS 124: George, have Be menstruating.
1986		Eble Campus Sl. Mar. 4: George – menstrual period: George is visiting.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 89: What’samatter, baby [...] you got a George?
2001		OnLine Dict. of Playground Sl. 🌐 george n. Word used to mean a menstrual period. At Ackworth School, Yorkshire (a Quaker boarding school) the dates of the girls’ periods had to be recorded in the ‘George Book’.