Hair washing and the liquid for it, from Hindi champo, the imperative of champna: to press, knead, massage. The word is literally a command: massage!
Documented
The imperative
Documented: Hindi champo, imperative of champna, to press or knead, enters English in the eighteenth century through the East India Company world, first meaning massage, the bath attendant's art. Sake Dean Mahomed, a Bengali surgeon, opened his shampooing bath in Brighton in 1815, became shampooing surgeon to George IV and William IV, and is honored in Brighton as the man who introduced shampooing and therapeutic massage to Britain.
Assessment: Documented: the Hindi verb, Mahomed's bathhouse and royal warrant. The word was a verb of service before it was a noun of shelf.
Stories
The product drift
The documented narrowing: by the nineteenth century shampoo meant washing the hair, then the soap for washing hair, then the category leader of the toiletries aisle. The massage evaporated; the command to press and knead survives on a bottle that no one kneads with anymore.
Assessment: Stories: every bottle of shampoo is an instruction manual that lost its verb. The imperative became the object.
Interesting claims
The empire in the aisle
The pattern: shampoo belongs to the great Indian influx into English, bungalow, pajamas, veranda, cot, loot, thug, juggernaut, words taken during the Company century along with everything else. The bathroom shelf is a small museum of the empire's vocabulary, most of it softened into domesticity.
Assessment: Interesting: the acquisition is documented in the lexicons. The softening is the empire's signature on the language.
Sources and further reading
Oxford English Dictionary, shampoo: from Hindi champo, imperative of champna, to press, knead
Sake Dean Mahomed, shampooing bath, Brighton, 1815; shampooing surgeon to George IV and William IV (documented royal warrant)
The nineteenth-century semantic shift from massage to hair-washing product (documented in the lexicons)