The phrase "built fire under" usually refers to a situation where someone has been motivated or encouraged to take action. Some antonyms for this phrase could include "quenched the fire," which means to suppress or extinguish the motivation, or "limp motivation," which indicates that the person is lacking energy or drive. Another antonym could be "demotivated," which means to cause someone to lose their motivation or enthusiasm. "Undermined" could also be an antonym for "built fire under," as it suggests the opposite of encouragement and motivation. These antonyms reflect a range of attitudes and emotions that can counter the effects of having a fire built under someone.