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.
frost up
v.
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.