In ancient times, a pook was believed a bad spirit. When things went wrong, however, you could save yourself from further exposure to such bad luck by the naming the spirit aloud. Unfortunately, as soon as you opened your mouth to name it, you ran the risk of the spirit entering your body and giving you permanent ill fortune. This could be countered by placing the fingers in the mouth prior to saying the name. Of course, our ancestors were'nt as keen on the anti-bacterial soap as we are, so stuffing filthy fingers in the mouth brought on more bad luck in the form of illness--clever folks they were though, they devised a way to pronounce the word 'pook' and make it sound as though the fingers were in the mouth when in fact they were not.