Watertight

adj.capable, thoughtfulprofessional

What it means

So well supported or constructed that no serious gap is apparent.

Reach for it when praising an argument, plan, or safeguard that has survived close challenge

The auditor built a watertight case from signed logs, timestamps, and independent records.

playful
ironclad
when certainty or protection should sound forceful
not when Ironclad is rhetorical and may overstate a case that remains open to revision.
everyday
airtight
when no logical opening or escape remains
not when Airtight is nearly interchangeable, but it sounds more idiomatic for alibis and logic than physical sealing against water.
everyday
foolproof
when a process prevents ordinary user error
not when Foolproof can demean users while leaving the underlying argument invalid.
everyday
sound
when dependable reasoning is enough without claiming perfection
not when Sound is the safer choice if small limitations remain.
professional
rigorous
when strict method and testing create confidence
not when Rigorous work can still reach a conclusion with acknowledged uncertainty.
professional
robust
when a system keeps working under stress or variation
not when Robust allows imperfections, whereas watertight claims no meaningful gap.
formal
conclusive
when evidence settles a disputed question
not when Conclusive applies to proof or results, not the design of a plan.
literary
unassailable
when no credible objection can defeat the position
not when Unassailable is grander and more absolute than most practical plans justify.

watertight argument
Her watertight argument accounted for every apparent exception.
watertight case
The investigators presented a watertight case without relying on rumor.
watertight plan
A watertight plan assigned a backup for each critical delivery.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.