wigless head; natural hair
Slang term for a head without a wig, or one's own natural hair as opposed to a wig. Often used in contexts where someone is normally seen wearing a wig.
He usually wears a wig, but today he came with his natural hair.
intelligence; smarts; comprehension
Slang for one's innate intelligence or mental capacity, often used in phrases like 地頭がいい (naturally smart) or 地頭が悪い (not naturally bright).
He's naturally smart, so he quickly understands even difficult problems.
Compound of 地 (ji, 'real' or 'natural') and 頭 (atama, 'head'). The exact origin is uncertain, but it likely arose as slang contrasting natural state with artificial (e.g., wig) or acquired (e.g., learned knowledge).