When we think of synonyms for the phrase "harboring a design," some common alternatives come to mind. These include terms like having an intention, a plan, or a motive. Other possibilities might include a hidden agenda, an ulterior motive, or a scheme. No matter what words we use, the meaning is clear: the individual in question has a specific objective in mind, which they are trying to conceal from others. Whether this is something sinister or just a harmless plan, the fact that it is being kept secret suggests that the person may be up to no good. As such, it's wise to be cautious around those who are harboring designs - you never know what they might be hiding.