What does Boxing Day have to do with boxing?
1660s, "to shoot too low," from under + shoot (v.). In reference to aircraft or pilots, recorded from 1918. Undershot as a type of water wheel is recorded from c.1600.
undershoot un·der·shoot (ŭn'dər-shōōt')n. A temporary decrease below the final steady-state value that may occur immediately following the removal of an influence that had been raising that value.