1. To check; keep in check (a fire).
2. To lessen in force or effect; SYN. damp, soften, weaken, break.
3. To quieten or silence (a sound) or make (an image) less visible; SYN. muffle, mute, deaden, damp, tone down.
4. To reduce the amplitude (of oscillations or waves).

To make (more) humid; SYN. moisturize.