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inefficace

ineffective

i . n ɛ . f i . k a s
in not; lacking in + efficace effective, efficient
adjective Rare #44,542
ineffective
FR: Cette méthode est inefficace contre les taches.
EN: This method is ineffective against stains.

C'est franchement inefficace.

It’s honestly ineffective.

Tu es inefficace.

You are inefficient.

Tom est inefficace.

Tom is ineffective.

Par chance, le traitement fut seulement inefficace plutôt que dangereux.

Luckily, the treatment was only ineffective rather than dangerous.