from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A defensive tactic in an espionage trial whereby the accused threatens to reveal secret information unless the charges are dropped.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The threatened exposure of state secrets in order to manipulate legal proceedings.
gray1 + (black)mail.(American Heritage® Dictionary of the English Language, Fourth Edition)
From gray + mail, by analogy with blackmail (Wiktionary)