What does the "Psi" in "Psi broth" stand for?
Psi is the Greek letter ψ.  Psi broth is sometimes referred to as ψ-broth.

Here's an example:


  L-Broth or ψ-broth can be used instead of SOC medium, but efficiency may
  be reduced.
  
  
  L-broth： 10 g Bacto tryptone, 5 g Bacto yeast extract, 5 g NaCl, pH to
  7.5 with 1 M NaOH, bring to a final volume of 1 L, and autoclave.
  ψ b-broth： 5 g Bacto yeast extract, 20 g Bacto tryptone, 5 g
  MaSO4･7H2O, pH to 7.5 with 1M KOH, bring to a final
  volume of 1 L and autoclave.