Final answer:
The correct word to describe the witness as a notorious liar in the given context is 'arrant.' 'Saturnine' does not fit the context of credibility and trustworthiness but refers to a gloomy demeanor.
Step-by-step explanation:
The defense attorney succeeded in portraying the genial witness as a(n) arrant liar whose testimony was never credible. The word 'arrant' means complete, utter, or in the context of the sentence, a notorious liar. The adjective 'saturnine' would not be appropriate here as it describes someone who is gloomy or morose, which doesn't relate to the witness's credibility. In legal contexts, establishing the credibility of witnesses is crucial, as seen in various historical court cases, including the Supreme Court of Herkimer County's 1914 verdict. Portraying a witness's testimony as untrustworthy can significantly affect the outcome of a case.