Quixotic

 "Mannix will bear looking into."

Markham put down his knife and fork and leaned back. I'm overcome! Such Himalayan sagacity! With that evidence against him, he should be arrested at once. Vance, my dear old friend, are you feeling quite normal? No dizzy spells lately? No shooting pains in the head? Knee jerks all right?"

"Furthermore, Dr. Lindquist was wildly infatuated with the Canary, and insanely jealous. Recently threatened to take a pistol and hold a little pogrom of his own."

"That's better." Markham sat up. Where did you get this information?"

"Ah! That's my secret."

Markham was annoyed. "Why so mysterious?"

"Needs must, old chap. Gave my word and all that sort of thing. And I'm a bit quixotic, don't y' know - too much Cervantes in my youth." He spoke lightly, but Markham knew him too well to push the question.

(from The "Canary" Murder Case, by S. S. Van Dine)