watertight

adjective

1 a watertight container:

waterproof, water-repellent, water-resistant, weatherproof;

damp-proof;

impermeable, impervious, sealed, hermetically sealed, coated, proofed, rubberized.

▷antonyms leaky.

2 Sam had arranged a watertight alibi:

indisputable, unquestionable, incontrovertible, undeniable, irrefutable, unassailable, impregnable, beyond dispute, beyond question, beyond doubt, indubitable;

foolproof, sound, flawless, airtight, conclusive, perfect, without loopholes.

▷antonyms flawed.

watertight Oxford Dictionary of English