大姑娘 is a warm, colloquial northern Chinese expression for a grown or maturing girl, often used affectionately by older relatives. It can also specifically mean 'eldest daughter' (in contrast to
二姑娘,
三姑娘). The phrase
大姑娘上轿头一回 (a girl getting into a bridal sedan for the first time) is a set phrase meaning 'a first-time experience.'
