means Describing something regarded as sacred, set apart for or belonging to a divine power — and, in casual speech, an intensifier for anything astonishing.

from From Old English 'halig,' meaning sacred or consecrated, which traces back to a Germanic root tied to the idea of 'whole' or 'unharmed' — so to be holy was, at heart, to be intact and inviolate. That same wholesome root is a cousin of 'whole,' 'heal,' and 'health,' meaning the language quietly links the sacred to the sound and unbroken. The exclamatory uses ('holy cow,' 'holy smokes') are far younger, mostly playful 20th-century dodges for swearing on something genuinely holy.