笄 (jī) is a hairpin. In ancient China, a girl's
笄礼 (jīlǐ, 'hairpin ceremony') at fifteen marked her as marriageable — her hair was put up and fixed with a
笄. The male equivalent was
冠礼 (guānlǐ, the capping ceremony) at twenty, behind
弱冠 (ruòguān, 'weak-cap age').