The Nancy Reagan Defense is a tactic in corporate finance used to counter a takeover or merger bidder who has made a formal bid to shareholders to buy their shares. When the board of directors of the target company meets to consider the bid, they "just say no."
"Hostile Tender Offers and the `Nancy Reagan Defense': May Target Boards `Just Say No'? Should They Be Allowed To?" Robert A. Prentice and John Langmore, 35 Delaware Journal of Corporate Law 377-481 (1990)