Holy and Whole Are Close Cousins From One Ancient Root
The words holy and whole share a single starting point, and the w sitting at the front of whole has no historical reason to be there.
Holy and whole look like two unrelated words doing two unrelated jobs. They are close cousins, and both come down from the same old word. That shared history also explains a puzzle that has annoyed careful spellers for a long time: the w at the front of whole has no business being there.
The stray letter in whole
Whole goes back to the Old English word hāl. Its relatives in other old Germanic languages include Gothic hails, Old Icelandic heill and Old High German heil, and not one of them begins with a w. Nothing in the entire history of this adjective justifies the initial letter. The likeliest reason for it is a practical one: it was perhaps added to keep whole apart from hole, a word it would otherwise have matched exactly in sound. The letter is a disguise, not a piece of the word's past.
Two words, one shape
Holy means the same as the Latin word sanctus, and it is related to English hallow as well as to whole. Put the two words side by side and they are almost identical twins. Whole goes back to a form that sounded like hail-az; holy goes back to one that sounded like hail-ig-az. These are not written records but the best guess of word historians at how a much older spoken word sounded before anyone wrote it down. The only difference between the two is an ending tacked onto holy, and it appears to have changed nothing about the meaning — a strange thing for such an ending to do, though nobody has found a way to work out what it once did. Take it away and both words arrive at the same starting point. The most ancient form of holy sounded approximately like hail-ag-az. That conclusion comes from a 1960 paper by Professor Gustav Must.
How healthy turned into holy
The word behind both holy and whole began with a meaning closer to healthy or complete. The step from there to sacred is not a long one: a word for whole and complete could easily come to mean morally and spiritually perfect, which is one step from holy. English kept the family together in other ways too. The verb heal means to make someone whole, and heal is related to holy. A line from the book The Kingfishers Catch Fire makes the point simply: "What is heal", asked Teresa. "It is to make whole", said Dr. Glenister. "Our Lord touched the lepers and blind men and made them whole."
Nothing shows that holy was used as a religious term in Germanic before Christianity arrived. Some researchers suggested the word had once meant inviolable; others rebuilt it as propitious, lucky, good. In the old Icelandic phrase holy gods, the sense was probably deserving worship, and it was only in the monotheistic Christian world that the word took on the meaning heard today: separated from sin, morally perfect. Germanic had a rival word for sacred as well — Gothic weihs — but it left no trace in Modern English. One scrap of it survives in the last letter of the Danish place name Odense, which meant Odin's sanctuary. The word English kept for holy is the one that had started out meaning healthy.