Weaken, v. t. [imp. & p. p. Weakened; p. pr. & vb. n. Weakening.]
1. 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. Their hands shall be weakened from the work, that it be not done. Neh. vi. 9.
2. To reduce in quality, strength, or spirit; as, to weaken tea; to weaken any solution or decoction.
1 other Webster entry under this spelling
Weaken, 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. "His notion weakens, his discernings are lethargied." Shak.
Shades of meaning
Fernald lists weaken only as an antonym, under confirm.
He lists it without comment; the entries above explain its neighbors.