noun

• (by extension) Compromising material that can be used to extort someone.
• (by extension) Compromising material that can be used to extort someone.
• (archaic) A form of protection money (or corn, cattle, etc.) anciently paid, in the north of England and south of Scotland, to the allies of robbers in order to be spared from pillage.
• (England law, historical) Black rent; rent paid in corn, meat, or the lowest coin, as opposed to white rent, which was paid in silver.

verb

• (transitive) To extort money or favors from (a person) by exciting fears of injury other than bodily harm, such as injury to reputation, distress of mind, false accusation, etc.
  "He blackmailed a businesswoman by threatening to expose an alleged fraud."
• (Kenya) To speak ill of someone; to defame someone.