The Collaborative International Dictionary of English v.0.48:

Hypo \Hy"po\, n.
   Hypochondria. [Colloq.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hypo \Hy"po\, n. [Abbrev. from hyposulphite.] (Photog.)
   Sodium hyposulphite, or thiosulphate, a solution of which is
   used as a bath to wash out the unchanged silver salts in a
   picture. [Colloq.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hypo- \Hy"po-\ [Gr. ? under, beneath; akin to L. sub. See
   Sub-.]
   1. A prefix signifying a less quantity, or a low state or
      degree, of that denoted by the word with which it is
      joined, or position under or beneath.
      [1913 Webster]

   2. (Chem.) A prefix denoting that the element to the name of
      which it is prefixed enters with a low valence, or in a
      low state of oxidization, usually the lowest, into the
      compounds indicated; as, hyposulphurous acid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hypo
    n 1: a compound used as a fixing agent in photographic
         developing [syn: hypo, sodium thiosulphate, sodium
         thiosulfate]
    2: a piston syringe that is fitted with a hypodermic needle for
       giving injections [syn: hypodermic syringe, hypodermic,
       hypo]