Underneath all its chick-flick gloss, Monster-in-Law is a pretty shrewd switched-sex spinoff from the Meet the Parents formula.

For a change, it’s boys on the side, comically crowded out by wildly inspired performances from Jane Fonda, Wanda Sykes, Elaine Stritch and, yes, even Jennifer Lopez — for whom you’d have to go back to pre-pop star days to find a performance this effortlessly appealing.

Putting the profanity in the film’s PG-13, Sykes reworks her shtick of annoyed commentary, but she’s the chorus the film needs. And how director Robert Luketic employs Stritch, who shows up for the film’s finale, is a textbook example of how grand dames of comedy should be used.

This quartet makes the movie, not gags about fish genitalia and dog sex, the prerequisite combination of a gay man and a girlfriend as the protagonist’s pals or allergies making someone’s face swell up — already an empty rom-com joke after this and Hitch.

Lopez stars as Charlie, a dog-walker, artist and temp who coincidentally runs into the same hunky doctor (Michael Vartan of Alias) several times in two days. Clearly, when Vartan’s Kevin offers a moment’s-notice dissertation on how the reflecting light changes Charlie’s eye color, this is a match made in meet-cute heaven.

Not so with Charlie and Kevin’s mom, Viola (Fonda), a semi-alcoholic four-time divorcee recently sacked from her newsmagazine gig for someone younger. Her on-set response (clothes-lining a Britney Spears look-alike) gets her sent in for rage-issue treatment. She seems OK, but when Kevin proposes to Charlie in front of Viola, it sparks a monstrously manipulative battle of wills.

Fonda hasn’t taken a film role in 15 years, and since she’s content just being Jane Fonda, Monster-in-Law is a comeback vehicle she doesn’t really need. But boy, does she floor it.

Hers is a boozy, wet-eyed unglamorous performance that’s both nervously riotous and subtly human. There are obviously campy bits (Viola’s garish costuming alone is worth several laughs), but there’s also the obvious, cutting way she briefly flaunts her cross pendant in front of Charlie, who prefers spirituality to religion.

Better yet, Fonda persuades us of what the script is too patchy about — that ageism has sent this hair-trigger mama over the edge with concern she’s losing even her loving son to this year’s model.

Lopez plunks fine grace notes of mounting frustration with this woman that are even better than the sassy zingers she fires at her. And when it literally comes to blows between the two, it’s a surprising moment born of true anger and not just the sudden need for lively slapstick.

Monster-in-Law could have sunk its claws deeper — slight intimations that Viola’s dislike of Charlie is tied to her Latina heritage are confined to jokes about her sizable backside. But it’s still the rare crowd pleaser that sacrifices neither its plot’s plausibility nor its actresses’ dignity for a joke.