The word grimace — meaning a facial expression of physical or emotional pain, disgust, or disapproval, or the act of making such a face — came into English in the seventeenth century from an alteration of the Middle French grimache. For the longest time, even into the early 1970s, the word was almost universally pronounced grih-MAYS, rhyming with face and with the accent on the second syllable. It was even the pronunciation William Burroughs used in 1993 when he recorded my favorite Christmas story, "The Junky's Christmas." (Here's a Spotify link.)
|And then, through a freak accident, Barney's giant pile|
of poo became sentient.
Why the folks at Mickey-D's decided to call him that, I can only guess. With its associations with pain, disgust, and disapproval, plus the added emphasis on the syllable GRIM, Grimace seems like the worst possible name for a character whose sole purpose was to convince parents that McDonald's is a great place for a family dinner.
Then again, it taught the whole world a good word to describe its reaction to the fast food giant's fare.