light head; vague head; fuzzy thinking
Describes a state of mental cloudiness or inability to think clearly, often from fatigue, illness, or intoxication. The expression is somewhat colloquial.
My head is fuzzy from the cold and I can't think straight.
I went to the meeting with a foggy head from lack of sleep.
From 濁る (にごる, 'to become muddy/cloudy') + 頭 (あたま, 'head'). The expression figuratively extends the idea of clouded liquid to a clouded mind.