Marcie calls Peppermint Patty "sir" for several reasons, though the exact reason is never definitively explained by the creator Charles Schulz. It may have started as a genuine accident or a habitual reaction to Peppermint Patty's strong, sometimes bossy, tomboyish personality. Marcie might have begun calling her "sir" out of admiration, respect, or to gently tease Patty, who often gets annoyed by it. One popular theory is that when they first met, Peppermint Patty was a camp counselor for Marcie, so Marcie called her "sir" out of respect. Over time, Marcie continued using it partly to irritate Patty. The dynamic reflects their unique friendship where Marcie's serious and studious nature contrasts with Peppermint Patty's tomboyish, athletic character. Despite the annoyance, they remain close friends, and Patty eventually grows accustomed to the nickname.