Noah Webster's New International Dictionary of the English Language

1. (v. t.) To make weak; to lessen the strength of; to deprive of strength; to debilitate; to enfeeble; to enervate; as, to weaken the body or the mind; to weaken the hands of a magistrate; to weaken the force of an objection or an argument.

2. (v. t.) To reduce in quality, strength, or spirit; as, to weaken tea; to weaken any solution or decoction.

3. (v. i.) To become weak or weaker; to lose strength, spirit, or determination; to become less positive or resolute; as, the patient weakened; the witness weakened on cross-examination.