let blood

verb3

1. •
   (figurative) To make (someone or something) bleed, in a general sense; to cut; to kill.
2. •
   (no longer used, historical) To extract blood from (a person, part of the body etc.).

   • Is the foole sicke […] Alacke, let it blood. — 1595
3. •
   (no longer used, historical) To bleed someone; to extract blood from a person, part of the body etc. for supposed therapeutic purposes, especially by phlebotomy.