big-girl pants

noun[plural only]2

1. •
   (informal) Pull-up underwear worn by a girl during and after toilet training, as contrasted to the diapers worn during infancy.

   • Talk about how she'll be able to wear big-girl pants or fancy pants (decorated training pants) because she will learn to use the potty. — 1987
2. •
   (informal, figurative) A notional pair of trousers or underwear worn by a female when acting maturely.

   • So put on your big-girl pants and own up to what you and your stupidity caused. — 2011