The antonyms for "pumping up" can vary depending on the context. If referring to inflating something, the antonyms can be deflating, decompressing, or lowering. In a fitness context, the antonyms can be resting, relaxing, taking it easy, or winding down. In a motivational context, the antonyms can be discouraging, demotivating or disheartening. In a musical context, the antonyms can be slowing down, toning down, or softening. Therefore, it is crucial to understand the context before determining the antonyms for "pumping up" to get a proper sense.