The phrase "went flatout" means to act with maximum effort or speed. There are many other synonyms for this phrase that can convey similar meanings, such as hustled, raced, hurried, sprinted, dashed, bolted, and galloped. These words can be used in a variety of contexts, from describing the speed of a marathon runner to the urgency with which someone completes a task. Depending on the tone and context of the situation, different synonyms may be more appropriate than others. However, they all share the common thread of indicating a rapid or intense level of activity.