Make heavy means to add weight or make something heavier. However, there are several synonyms that could be used to describe the same thing. One synonym for make heavy is to weigh down, which means to add weight or burden. Another synonym could be to load up, which means to add a heavy amount of something to something else. You could also use "burden", "encumber", or "weigh upon" as synonyms for make heavy. These synonyms are useful when you need to describe an object, substance, or situation that has a significant weight or impact on its surroundings. Regardless of which synonym you choose, the meaning is the same: something is being made to be heavier and more difficult to move or handle.