The word frivolous is thrown around way too often in legal contexts, and usually is used inaccurately.
In everyday life, to call something frivolous is essentially to call it unserious, e.g., “stop with the frivolous pursuits and get a job.” But in the world of litigation, it means something different, because most legal disputes are over shades of gray.
Just as an example, suppose two sides are fighting over whether a particular expert should be allowed to testify at trial. Each side is likely to articulate a position, citing to law, as to why their desired outcome is correct. Because the court is going to resolve this issue, one side’s argument is going to be more convincing than the other’s (perhaps significantly so), but it’s rare that the losing side had no point whatsoever.
Too often, people in litigation label something frivolous when what they really mean is “a weak argument/one I don’t like.” Frivolousness is actually reserved for these rare occasions in which a filing is devoid of merit and is typically brought for an improper purpose, such as to harass or delay.
So it’s not just bringing a lawsuit/motion that ultimately is a loser, or even claims that are dubious. Rather, it’s bringing a suit (or filing an Application) that self-evidently has no basis to exist.