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Frivolous means silly, and usually also wasteful. Then we get into the legal meaning.
Frivolity is a concept with two meanings when discussing the law. One definition applies most notably to personal injury lawsuits with seemingly outrageous payouts. Corporations would love to see these lawsuits reduced, so they are often fans of "tort reform", which generally amounts to restricting people's rights to sue. Trial lawyers hate this because it takes away from business, and consumer advocates hate this because it denies a victim of negligence the right to redress of grievances. Yes, such lawsuits can be used to game the system, but really, that's a different subject.
"Frivolous" also has a meaning as a legal term of art. Dan Evans' Tax Protester FAQ lays it out fairly bluntly — if a judge calls a lawsuit or argument frivolous, it is in the judge's opinion a blatantly false (or not even wrong) argument and a waste of the court's time. A lawsuit deemed frivolous in the public media merely gets ridicule; a frivolous lawsuit in a courtroom can lead to contempt citations and fines or jail time. (Cases with no merit that aren't totally stupid can be dismissed, without necessarily being labeled frivolous.)
Make sure you know the difference. Hint: if it's a reporter saying it, feel free to argue the case. If it's a judge saying it, shut up, apologize, and walk away.