What does "scream bloody murder" mean?

Angrily protest as loudly as possible, as in When Jimmy took her teddy bear, Lauren screamed bloody murder, or Residents are screaming bloody murder about the increase in property taxes. The scream here may be either literal (as in the first example) or figurative, which is also true of invoking murder as though one were in danger of being killed. Versions of this term, such as cry murder, date from the 1400s.