The name comes from the sherry oak cask where the whisky is kept to mature. Now, we know oak, which is a tree. Is ‘sherry’ a type of oak? No, it isn´t.
It turns out sherry is a type of wine, yeah, and it comes from Spain. It’s what is called ‘vino de Jérez’, as it comes originally from Jérez. So a sherry oak cask is the oak cask used to keep sherry wine. And these casks are reused to keep the whisky.