The adjective fallacious describes something that involves a fallacy or tends to deceive or mislead.

Synonyms are delusive, fictitious, fraudulent, or unreasonable.

The word origins from the Latin word fallāciōsus meaning “deceitful or deceptive” and from the Old French fallacieux. Its first recorded use was during the 15th century.

Everything he said in court was fallacious.

I don’t understand why he always says fallacious and unreasonable things, I mean it doesn’t hurt to be honest for once, right?

Fallacious testimonies can be easily defeated by strong evidences.