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
My daughter has started wanting to wear big girl panties.
Once toilet training progresses, let's switch to big girl panties.
Compound of お姉さん (older sister, big girl) and パンツ (panties). The term likens the child to an older, more independent girl who no longer needs diapers.