The phrase is often used to describe problems caused by cold weather on objects. The lock will ice up if the temperature drops overnight.
Pro Tip 2/3
Safety Concern
When surfaces ice up, it usually indicates a potential hazard, like slippery roads or walkways. Be careful walking to school; the sidewalk might ice up.
Pro Tip 3/3
Visibility Issues
Windows icing up can reduce visibility, mainly in vehicles or buildings. I had to wait for my car windows to defrost because they iced up overnight.