young woman who knows a lot about sex from hearsay but lacks experience
Often used critically or humorously for a young woman who has picked up sexual knowledge from conversations or media but is still inexperienced. The term can sound dated or mildly derogatory.
She's a 'mimidoshima' — she talks a good game about sex but doesn't actually know anything from experience.
Compound of 耳 (ear) and 年増 (middle-aged woman). The term suggests a young woman who has the worldly knowledge of an older woman, but only through hearsay.