Sham-poo. Sham as in fake? Poo as in, well, poo?
Well, as you know, that's not it at all.
And since I am sure that you are all agog to know the origin of the word, here it is:
From Dictionary.com: 1755–65; earlier "champo" to massage < an inflected form of Hindi cāmpnā literally, to press.
And from Wikipedia: The word shampoo in English is derived from Hindi chāmpo (चाँपो [tʃãːpoː]), and dates to 1762. The Hindi word referred to head massage, usually with some form of hair oil.
So, there you have it, dear reader! Trivia to start your weekend.