union suit

/Y UW1 N Y AH0 N S UW1 T/
noun
  1. 1

    One-piece underwear, usually long, combining drawers and long-sleeved, button-front undershirt in a single garment, with a flap in the seat that can be opened for defecation.

Source: https://en.wiktionary.org/wiki/union%20suit