What is an anti-SLAPP motion and can it get a defamation lawsuit thrown out early in Ontario?

Yes. Ontario has a specific anti-SLAPP mechanism built into the Courts of Justice Act that allows a defendant to bring an early motion asking a court to dismiss a lawsuit, including a defamation claim, on the basis that it improperly targets expression the defendant made on a matter of public interest. SLAPP stands for strategic lawsuit against public participation, describing litigation whose real purpose is seen as silencing or punishing public criticism or debate, rather than genuinely remedying a proven harm.

If the motion succeeds, the underlying lawsuit is dismissed before it proceeds through the normal, lengthy litigation process, which is the whole point of the mechanism - giving defendants facing this kind of claim a way to have it screened out early rather than being forced through extensive and expensive litigation. The motion involves its own specific legal test, addressed in a related question, and is not available for every defamation claim - only for ones that genuinely involve expression on a matter of public interest in the way the legislation defines that term.