noun
big girl panties; girls' underwear (post-diaper)
Child-rearing term for girls' underwear worn after toilet training; often used in the context of transitioning from diapers. Compare トレーニングパンツ (training pants).
See also: トレーニングパンツ
娘がお姉さんパンツを履きたがるようになった。
My daughter has started wanting to wear big girl panties.
トイレトレーニングが進んだら、お姉さんパンツに切り替えましょう。
Once toilet training progresses, let's switch to big girl panties.