'Foggy' has been used with reference to people's brains since the 1600s. The suggestion is that the ideas in it are murky and unclear. As we use it today, it is just an abbreviated form of "I haven't the foggiest idea"...that is, I have no clear idea at all about the subject.
I told you, J...the Force is with me in the synchrography stakes at the moment! Not to worry, though; I've no doubt things will return to 'normal' soon.