To make something weaker or to reduce its intensity (deliberately); a transitive verb used for weakening force, sound, flavor, etc.
風の音がうるさいので、窓を少し閉めて音を弱めた。
The wind noise was loud, so I closed the window a bit to reduce the sound.
医師は薬の量を弱めることにした。
The doctor decided to reduce the dosage of the medicine.
相手の主張を弱めるために、具体的なデータを示した。
I presented concrete data to weaken the opponent's argument.