The guillotine. A tricoteuse would hang out in the Place de la Révolution during the Reign of Terror and knit while watching heads roll. (Now there’s an app for that.) Madame Defarge is a famous example, but encoding messages in their knitting is not a requirement.
Natalie Toro as Madame DeFarge, with company, in A Tale of Two Cities. (photo by Carol Rosegg)