The phrase "did utmost" refers to putting in maximum effort towards a particular task or goal. Its antonyms, on the other hand, mean the opposite - to not put in much effort or to do the minimum required. Some of the antonyms for "did utmost" include "slacked off," "backed off," "gave up," "relaxed," "slipped," "abandoned," "neglected," "lazied," and "skimped." These words indicate a lack of motivation or determination to accomplish something. While it's important to prioritize rest and not burn out, consistently doing the bare minimum can lead to a lack of progress and ultimately hinder success.