X-Waters An ancient name for distilled spirits in Ireland.
X4 A quadruple distilled whisky, also known as "Perilous Whisky", marketed by Bruichladdich from Islay
In 1695 a Hebridean traveller, Martin Martin, wrote of an ancient, powerful spirit known as “usquebaugh-baul” (oosh-ker-vah-voll), the Gaelic for ‘perilous whisky’. He was told:

 "…the first taste affects all the members of the body: two spoonfuls of this last liquor is a sufficient dose; and if any man should exceed this, it would presently stop his breath, and endanger his life.” 

It's fearsome reputation came from a unique quadruple distillation - the extra two distillations increasing the purity of the spirit - raising the strength to over 90% ABV.