If you're wondering whether Grandma has visions of you working tiny little knitting needles to connect your eyebrows into a scarf to wear right there on your face, think again. She's talking about how you express worry by working your facial muscles into a frown.
The word knit means bringing something closer together. We usually picture knitting as the way to make a piece of clothing, like a winter hat or a baby blanket.
You don't really need those special skills, though, to knit in different ways, like knitting together the right words to make an understandable sentence. You also could be a member of a closely knit family, a tight-knit social group, or a close-knit work team.
Try this: Make your mouth frown. Now squeeze your eyebrows together. You've knitted your brows.
Now, smile, while your try to knit your brows. Not so easy, huh? (And way ugly, anyway.)
The brow knitting gesture is much older than even your grandma. The book Madame Bovary, by Gustave Flaubert, was first published in 1857. First considered scandalous, the 19th-century work later became the model of a realistic novel.
Here's a passage from the book that shocked the world with its immorality:
In the midst of the silence that hung over the village a heart-rending cry rose on the air. Bovary turned white to fainting. She knit her brows with a nervous gesture, then went on. And it was for him, for this creature, for this man, who understood nothing, who felt nothing!