To contradict, to provide proof that something is not.
Example: "The thought is that you see an episode of observation, experiment, or reasoning as confirming or infirming a hypothesis depending on whether your probability for it increases or decreases during the episode."
Weak or ill, not in good health.
Example: "He was infirm of body but still keen of mind, and though it looked like he couldn't walk across the room, he crushed me in debate."
Irresolute; weak of mind or will.
Frail; unstable; insecure.