To drench something is to get it thoroughly wet. You might drench your sister with the garden hose to pay her back for squirting you with her water pistol.
When you're sailing a small boat on a stormy day, waves might drench you, and a torrential rain storm can also drench you, if you leave your umbrella at home. If your dog is crazy about swimming, he might drench himself often by plunging into whatever body of water you're near. Drench comes from the Old English drencan, which means both "submerge or drown" and "give drink to or make drunk."