Green’s Dictionary of Slang

the contraction of the vaginal mucles around the ejaculating penis, thus increasing the sensation.

1898

‘Suzan Aked’ The Simple Tale of Suzan Aked 76: Your cunnie, if it has the nutcrackers, tightens and loosens on his prick, giving him further delight [...] The nutcrackers [...] are when your cunnie grasps his prick [...] It must be the muscles about the narrow entrance that do it.