The phrase "letted up on" refers to easing or reducing pressure, restrictions or criticism. The antonyms for this phrase could include increase, ramp up, intensify, harshen, or tighten. While "letting up" can signify relief, a different perspective can frame it as a reduction in control, while its antonyms can conjure the sense of pushing forward with more effort or a greater degree of scrutiny. It is important to consider the context of one's usage of "letted up on" and its related antonyms, as the appropriate term will depend on the tone and intent of the conversation or situation.