The Oxford Thesaurus An A-Z Dictionary of Synonyms - blackmail n.

1) extortion, ransom, tribute, US graft: Even if you pay the blackmail, that is no guarantee that he won't demand more later. --v. 2) extort money from; force, coerce, compel, make: They had discovered his indiscretions and were blackmailing him. He blackmailed her into signing the alimony settlement.