airtight

adjective
  1. 1

    Impermeable to air or other gases.

  2. 2

    Having no weak points or flaws.

    โ€œWe have an airtight argument they wonโ€™t be able to punch holes in.โ€

  3. 3

    (of a person) Highly reserved in some matter, particularly tight-lipped or tight-fisted.

Source: https://en.wiktionary.org/wiki/airtight