big boy pants

Noun

big-boy pants pl (plural only)

1. (informal, plural, plural-only) Pull-up underwear worn by a boy during and after toilet training, as contrasted to the diapers worn during infancy.
   • Taking the baby from Kidd she snatched the screaming Davey by the wrist and half-carried him up the stairs yelling, "You wanna wear big-boy pants? Big boys wear big-boy pants, but big boys use the potty!"1983, Randall Wallace, So Late Into the Night, Doubleday, published 1983, →ISBN, page 29
2. (figuratively, informal, plural, plural-only) 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, David H. Hoffman, "Ugly election rhetoric" [letter to the editor], Suburban Life, 25 May 2011, page A8

   Synonyms:

   big-boy britches

   Antonyms:

   bitchy pants, grumpy pants, scaredy pants