1. To become coated with a thin layer of ice. If the ground frosts up tonight, we'll have a much harder time digging into it tomorrow.
2. To cause something to become coated with a thin layer of ice. In this usage, a noun or pronoun can be used between "frost" and "up." I'm worried that the cold temperatures tonight will frost the ground up.
1. To become covered with frost: The windows frosted up quickly when the temperature dropped.
2. To cause something to become covered with frost: The freezing air frosted up the windows. Our breath frosted the mirrors up.