How to Say "quack" in Spanish
The Spanish word for “quack” is “charlatán” — B2 level.
English → SpanishB2
nounB2
a person claiming to have medical skills they don't have

Examples
Ese supuesto médico resultó ser un charlatán sin título.
That supposed doctor turned out to be a quack without a degree.
No compres ese tónico; el vendedor es un charlatán de feria.
Don't buy that tonic; the salesman is a fairground fraud.
Muchos charlatanes prometen curas milagrosas en internet.
Many frauds promise miracle cures on the internet.
Stronger meaning than 'hablador'
While 'hablador' just means talkative, calling someone a 'charlatán' in this context means they are lying or trying to trick you.
Not just 'chatting'
Mistake: “Using 'charlatán' to mean a 'chat' or 'conversation'.”
Correction: Use 'charla' for a chat. 'Charlatán' refers to the person, not the talk itself.
Related Translations
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