Know that everyone claiming this is a language or translation mix up (while conveniently ignoring your clarification that you and your MIL are both US English speakers) is gaslighting you. Maybe they just don’t want to believe Grandma is looking forward to hurting the baby to punish it for not adequately entertaining her? In any event, don’t leave her alone with the baby. She’s clearly told you she won’t respect your wishes or your helpless baby’s humanity.