The word "letting up on" refers to reducing pressure or easing off a situation, and its antonyms are words that express intensification or an increase in pressure. Some of the antonyms for "letting up on" include ramping up, tightening up, cranking up, stepping up, intensifying, increasing, and heightening the pressure. These words imply that the pressure or intensity of a situation is not decreasing but rather increasing in intensity. The antonyms for "letting up on" can be used to emphasize the need for more action or follow-through, particularly in difficult situations that require persistence and determination to overcome.