To cover or become covered with or as if with fog. The windows fogged up due to the cold weather outside.
To make something unclear or confused. The complex instructions fogged my understanding of the task.
To cause photographic film or paper to become veiled or hazy through accidental exposure or chemical error. Leaving the film near the window light fogged the entire roll.
To fill a space with a fine aerosol of insecticide, disinfectant, or similar substance. The pest-control crew fogged the warehouse to eliminate mosquitoes.