sheltered young lady; closeted maiden
Refers to a young woman from a wealthy, protective family, raised away from the outside world. Often carries a slightly old-fashioned or literary tone.
See also: 深窓
She grew up without any hardships as a sheltered young lady.
She, who had been a closeted maiden, suddenly had to go out into the world.
Also means a sheltered girl, but 箱入り娘 is more colloquial and emphasizes being kept at home like a treasure in a box, while 深窓の令嬢 has a more literary, elegant nuance of a high-class upbringing.
Compound of 深窓 (deep window, implying a secluded inner room) and 令嬢 (young lady). The phrase evokes the image of a daughter raised in the inner quarters of a wealthy household, away from public view.