Airtight means sealed so completely that no air passes. An airtight container, airtight seal, keep it airtight.
In storage it is what keeps food fresh and damp out, and an airtight container is the standard instruction on anything that goes stale. Figuratively it describes an argument or an alibi with no gap in it: an airtight case, an airtight alibi — which is much the commoner use in speech. Note watertight, which does the same figurative job in British English and is preferred there.
1adjectiveairtightSealed so completely that no air passes.
“Keep the biscuits in an airtight tin so they stay crisp.”
2adjectivegapWith no gap in the reasoning.
“The prosecution had an airtight case, with fingerprints, video and a confession.”
airtight is a quality of sealing.
Air plus tight, a plain nineteenth-century compound. Watertight is older and did the figurative work first, which is why British English still prefers it.
From air + tight.
Etymology from Wiktionary, CC BY-SA
Practised in Containers & Packaging