a thick cloud of very small drops of water in the air close to the land or sea that is very difficult to see through

a state in which things are not clear and seem difficult to understand 

He went through the day with his mind in a fog.

if a glass surface fogs or is fogged up, it becomes covered in steam or small drops of water so that you cannot see through

to make somebody/something confused or less clear

I tried to clear the confusion that was fogging my brain.

The government was trying to fog the real issues before the election.

LEARNit Dictionary