Plied is a verb that refers to the act of working diligently or continuously at a task. There are several synonyms for this word that could be used in its place, depending on the context of the sentence. Some examples include "labored," "plied," "toiled," "slogged," and "drudged." All of these words suggest a sense of persistent effort and hard work. Other alternatives might include "chiseled away," "hammered away," and "grinded away," which all suggest a more intense and focused effort. Ultimately, the choice of synonym will depend on the specific meaning and tone you want to convey in your writing.