noun

• (obsolete) Melancholy; a fit of hypochondria; a morbid depression.
  "1711 "I have a better Stomach tha usuall and have perfectly forgot what the Hyppo means",Joseph Collett, merchant, writing from Rio de Janeiro, Oct 15, 1711, in his Private Letter Books, edited by H. H. Dodwell in 1933. (cited by Eric Partidge, A Dictionary of Slang and Unconventinoal English"
• (slang) A hypochondriac.
  "Certainly no one can accuse a bunch of hypos of lying dormant during the noon-hour. The activity, inside and out, is simply terrific. To us, personally, by far the most interesting group of hypochondriacs in attendance at this particular emporium […]"
• (photography, informal) Sodium thiosulfate (also called hyposulfite of soda), a photographic fixing agent.
• (slang) A hypoglycaemia attack in a person with diabetes.

verb

• (intransitive, slang) To have a hypoglycaemia attack.
  "My top lip tingles, and I'm clearly hypoing but I don't know what my sugar level is, it was 9.8 when I went to bed, which is fine. I've hypoed a lot lately."
• (transitive) To administer a hypodermic injection to.
  "Bacterium and virus count—as usual. Lots of it. Apparently not too dangerous. Not with every single soul hypoed and immunized and hormoned to his eyebrows."
• (transitive, figuratively) To stimulate or boost, as if by administering a hypodermic injection. (Compare hype.)
  "Stations across the country hypoed the Beatle excitement with offers of fan club cards, free records. Beatle contests of all sorts, and a continual push on the Capitol, Swan, Vee Jay and MGM releases by the group."
• (transitive, economics) To stimulate or boost (the economy) by the injection of cash, such as with quantitative easing
  "In the last three peacetime years before the Vietnam War expenditures began to hypo the economy, 1961, 1962, and 1963, unemployment in those years averaged six percent."