The Collaborative International Dictionary of English v.0.48:

Shampoo \Sham*poo"\, v. t. [imp. & p. p. Shampooed; p. pr. &
   vb. n. Shampooing.] [Hind. ch[=a]mpn[=a] to press, to
   squeeze.] [Writing also champoo.]
   1. To press or knead the whole surface of the body of (a
      person), and at the same time to stretch the limbs and
      joints, in connection with the hot bath.
      [1913 Webster]

   2. To wash throughly and rub the head of (a person), with the
      fingers, using either soap, or a soapy preparation, for
      the more thorough cleansing.
      [1913 Webster]