intransitive. To become thick, in various senses; = thicken, v. Now dialect or archaic.

intransitive. To become dark, obscure, or opaque; of the weather: to become misty.

intransitive. With up: to become foggy or hazy.

intransitive. Of rain, snow, etc.: to fall; (of fog, mist, darkness, etc.) to descend, settle.