big-boy pants

noun[plural only]2

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

   • "And so," you continue, "I'm putting your diapers away because you can now wear big boy pants and do your poop and pee-pee in the potty chair all by yourself." — 1987
2. •
   (informal, figurative) A notional pair of trousers or underwear worn by a male when acting maturely.

   • If Horwitz is planning on running for office again in the future (and it certainly looks that way), perhaps it's time he put on his big-boy pants and engage in civil discourse. — 2011