Prim people, and their prim faces, voices, and behaviors, are polite and proper in a very careful way that can seem too formal or too cutesy, even goody-goody.
Here's the prim Professor Umbridge, from Harry Potter and the Order of the Phoenix. Harry immediately dislikes her. Her prim manner of speaking, the prim way she clears her throat before speaking ("hem, hem"), and her prim style of dress all just gross him out. She presents herself like a sweet, innocent young girl—all prim and proper—but she's actually pure evil.
So, that's the worst kind of primness: when someone acts primly, all dainty and cutesy, in a fake way that makes you want to barf.
But here's the best kind of primness: when someone acts primly, all wholesome and adorable, in a genuine way that makes you smile. Here's a prim young girl, Primrose Everdeen. "Prim" for short, she's Katniss's little sister in the Hunger Games series, and she embodies the good side of primness: she's neat, trim, tidy, clean, kind, gentle, and innocent. In Katniss's words, Prim's face is "as fresh as a raindrop," and "no one can help loving" her.
Remember: prim people, manners, and things are overly proper and polite, or extra-fussy about all the details, or super-duper clean and tidy.
Idea 1: "With an upturned nose and a prim little smile, (someone) said, 'We aren't allowed to (do something slightly fun, silly, or messy)."
Idea 2: "(Someone) was sitting primly, straight and tall, with (his or her) hands folded in (his or her) lap, as if (he or she) would never (do or say something wild, funny, or against the rules)."
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