The phrase "built a fire under" implies motivation, urgency or excitement to get something done quickly. However, there are several antonyms that can be used to indicate the opposite. Words like "lax", "lazy", or "apathetic" could describe a lack of motivation. "Uneager" or "reluctant" could suggest a hesitation to take immediate action. "Slothful" or "indolent" could imply a lazy disposition. "Passive" or "docile" could suggest a lack of urgency. "Uninterested" or "disinterested" could imply a lack of excitement for the task at hand. These antonyms provide a contrast to the vibrant expression of "built a fire under," indicating a lack of urgency or motivation.