To weaken is to become less powerful or strong. When a storm weakens, the wind dies down and the torrential rain becomes a light drizzle.
If you weaken physically, your strength diminishes: "I did great at the start of the obstacle course, but my arms began to weaken toward the end and I couldn't climb that last wall." You can also weaken in resolve or spirit, like when your sister's criticism of your art project weakens your confidence about including it in an exhibition. This verb comes from weak and its Germanic root meaning "yield" or "soft."