Definition of "grandmammy"

[…] and his orchestra as enchanting to dance to as it is, even if all this wasn't so superb, you'd want to go just to hear and see that grandmammy of all blues-singers who can still out-blue all of the younger ones, none other than Sophie Tucker.

1938, Stadium concerts review, volume 21

[…] that these administration signals took us students by surprise would be the grandmammy of all understatements. Folks were, depending on their inclinations, in turn suspicious, flattered, surprised, confused, or all of the above simultaneously […]

2003, Stokely Carmichael, Michael Thelwell, Ready for Revolution: The Life and Struggles of Stokely Carmichael (Kwame Ture), Simon and Schuster, page 222