From The First Amendment Encyclopedia:
A SLAPP suit, or strategic lawsuit against public participation, is a civil claim filed against an individual or an organization, arising out of that party’s speech or communication to government about an issue of public concern. At the heart of the SLAPP suit is the petition clause of the First Amendment.
‘SLAPP’ was coined to recognize lawsuits filed to silence criticism
A SLAPP suit may look like a civil lawsuit for defamation, nuisance, interference with contract, interference with economic advantage, or invasion of privacy, but its purpose is different. About this purpose, Judge J. Nicholas Colabella wrote in Gordon v. Marrone (N.Y . 1992), “Short of a gun to the head, a greater threat to First Amendment expression can scarcely be imagined.”
Professors George W. Pring and Penelope Canan coined the term SLAPP suit in the 1980s after noting a surge in lawsuits filed to silence public criticism by citizens.
SLAPP suits arise when citizens erect signs on their own property, speak at public meetings, report violations of environmental laws, testify before Congress or state legislatures, or protest publicly, among many other similar acts, thereby prompting a party who claims to be aggrieved by such acts — often developers, merchants, and even public officials — to file suit.
SLAPP suits can interfere with First Amendment rights
The petition clause of the First Amendment guarantees, in part, “the right of the people. . .to petition the government for a redress of grievances.” The abridgment of this right distinguishes a SLAPP suit from other cases based on similar allegations.
Defendants in SLAPP suits who plead a defense of petition clause immunity will almost always succeed in having the claims dismissed. This immunity is often known as the “Noerr-Pennington immunity” based on its role in Eastern Railroad Presidents Conference v. Noerr Motor Freight, Inc. (1961) and United Mine Workers v. Pennington (1965).
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Nonlegal effects of SLAPP suits remain
However, the nonlegal effect of SLAPP suits remains. A defendant in such a suit may succeed legally but lose nevertheless, having expended large amounts of time and money in defending against the lawsuit.
More damaging is the effect that such suits can have on those who have not yet been targeted: the desire to avoid being sued translates into a reluctance to participate in public debate.
Link to the rest at The First Amendment Encyclopedia