noun

• (figuratively) A person who derives advantage from others in a parasitic fashion.
  "'Wrecked his body and his mind, no use to hisself or his family or nobody, just a leech on society'."
• (medicine, dated) A glass tube designed for drawing blood from damaged tissue by means of a vacuum.

verb

• (transitive, literally) To apply a leech medicinally, so that it sucks blood from the patient.
  "The poppy made him sleep and while he slept they leeched him to drain off the bad blood."
• (transitive, figuratively) To drain (resources) without giving back.
  "Near-synonyms: mooch, suck down"

noun

• (archaic) A physician.
  "Many skillful leeches him abide to salve his hurts."
• (Germanic paganism) A healer.
  "Their functions are threefold, those of the medicine-man (the leech, or healer by supernatural means); of the soothsayer (the prophet through communion with the invisible world); and of the priest, especially in his capacity as exorcist"

verb

• (archaic, rare) To treat, cure or heal.
  "1564, Accounts of Louth Corporalː Paid for leeching.. my horses very sick."