undersuit

From under- + suit.

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undersuit (plural undersuits)

1. A soft undergarment worn by divers and astronauts.
   • 2006 December 8, “All you need to know about: Scuba diving”, in Alan Rusbridger, editor, The Guardian‎^[1], London: Guardian News & Media, →ISSN, →OCLC, archived from the original on 28 September 2014:

     If you plan to dive all year round in the UK, you'll need a drysuit rather than a wetsuit. It won't let in water (a wetsuit traps a layer of water against the skin), but doesn't keep you warm, so you need a neoprene or membrane undersuit, too.

2. Any suit worn under another suit; a suit of underclothes.