真空

zhēnkōng
  1. (physics, fig.) vacuum
  2. (slang) to not wear underwear

CEDICT

physics slang

Entry Composition

1 真 zhēn
  1. really
  2. truly
  3. indeed
  4. real
  5. true
  6. genuine
2 空 kōng
  1. empty
  2. air
  3. sky
  4. in vain