1. Of rain, to fall very heavily. It's been lashing down for the last hour or so. It's dry at the moment, but with how dark those clouds are it looks like it could lash down at any moment.

2. To tie or strap someone or something down very tightly. In this usage, a noun or pronoun is used between "lash" and "down." Make sure to lash that timber down before we start driving, or else it will all spill out onto the road. They lashed the prisoner down to the table and began torturing him to extract information.