inside of a helmet; hidden circumstances
Literal meaning is the interior of a helmet. Figuratively, it refers to hidden or private circumstances, often used in the idiom 内兜を見透かされる (to have one's hidden intentions seen through).
Sweat had accumulated inside the helmet.
He was at a loss for words when his hidden motives were seen through.
Standard kanji spelling.
Rare variant using 冑 (helmet).
Compound of 内 (uchi, 'inside') and 兜 (kabuto, 'helmet'). The figurative sense likely developed from the idea of the unseen interior of a helmet.