A “just say no” defense is a type of anti-takeover strategy used by boards of directors to fend off hostile acquisitions. The approach seeks to ensure that the board has the authority to evaluate a takeover bid and reject any offer that it deems unrealistic or disadvantageous for the company.

The strategy takes its name from Nancy Reagan's "Just Say No" movement against illegal drug use in the 1980s. It places the decision-making power in the hands of the directors, who can take a no-tolerance stance towards certain types of corporate takeover bids through a combination of policies and legal guidelines.

The idea behind the “just say no” defense is to discourage hostile takeovers by weakening the bidding company’s incentives. Companies that are intent on taking over a target company may also have to consider the possibility that the board of directors will reject their takeover proposal and make the process more difficult.

The legality of a “just say no” defense is largely dependent on the context of the situation. It can be difficult to tell if the defense is sufficiently motivated by the target company's legitimate long-term interests or if the board is using it to block a bid simply because it is deemed too low. Courts in some cases have found that the “just say no” defense is legal as long as it is used without the intent to suppress a takeover for selfish reasons.

In addition to the just say no defense, companies also use a number of other strategies to discourage hostile takeovers, including the poison pill strategy and the white knight strategy. The poison pill strategy can be used to make a hostile takeover prohibitively costly by granting additional voting rights to existing stockholders. Moreover, a white knight strategy is an effort to obtain a “friendly” bidder who would provide both a better bid and better terms for the target company.

Overall, the just say no defense is an effective tool for boards of directors to exert control and evaluate takeover offers on the target company's behalf. Although legalities may vary depending on the context, the defense can be a useful way of encouraging better offers or even finding a white knight bidder.