nighttime diaper

noun2

1. •
   (US, Canada) An extra-bulky and absorbent diaper which is intended to be worn during sleep for prolonged wetting, usually by an infant who is a heavy wetter, or by someone with urinary incontinence.

   • Old flannel receiving blankets are useful as nighttime diapers for Baby, being soft and so absorbent. — 1966
2. •
   (US, Canada) A diaper or training pants intended to be worn during sleep to prevent bedwetting, often by young children.

   • Since most children will sty dry in the day some months before they begin staying dry all night, it seems sensible to wait until they are dry during the day before abandoning their nighttime diapers. — 1967