Duck ague, also buck fever or buck ague, is a hunting term in which a marksman or hunter, before taking a shot with either a gun or bow in a tense situation, loses mental quietude and misses the shot.
In the movie Deliverance, Jon Voight suffers from duck ague before shooting a wild deer, after which Burt Reynolds' character describes the phenomenon, saying "Hell, I've known tournament archers, damn good shots, never out of the five ring. Draw down on a live animal, they get buck ague."
References
Duck ague Wikipedia(Text) CC BY-SA