steam up

verb5

1. •
   To become steamy.
2. •
   To become coated with condensation.

   • The bathroom mirror steamed up when he opened the shower door.
3. •
   To cook by steaming.

   • I'll steam up a bunch of dumplings.
4. •
   To bring (a steam boiler) up to working steam pressure: to build up a head of steam inside (a boiler).

   • It takes about an hour to steam up the locomotive.
5. •
   to build up a head of steam.

   • Engine 99 had been steaming up since 21:00.