What does Boxing Day have to do with boxing?
c.1600, "disprove, confute," from French évincer "disprove, confute," from Latin evincere "conquer, elicit by argument, prove," from ex- "out" (see ex-) + vincere "overcome" (see victor).
Meaning "show clearly" is late 18c. Not clearly distinguished from evict until 18c. Related: Evinced; evinces; evincing.