Grimace was originally associated with stealing milkshakes in McDonald's advertising.
When Grimace debuted in 1971, he was presented as a large, purple character with four arms who took milkshakes. McDonald's later revised the character, reducing him to two arms and giving him a gentler, more child-friendly personality.
The redesign helped Grimace become a friend rather than a threatening or mischievous figure. He developed into one of the central members of McDonaldland, appearing alongside Ronald McDonald, the Hamburglar, Birdie, and other branded characters.
A common mix-up is to remember Grimace only as the purple mascot seen in later decades and overlook his original advertising role. The early milkshake-stealing concept belongs to the character's first version. The Hamburglar, by contrast, was the character specifically associated with trying to take hamburgers.