What does Boxing Day have to do with boxing?
1850, for Henry Deringer (1786-1868), U.S. gunsmith who invented it in the 1840s; prevailing misspelled form is how his name appeared on the many counterfeits and imitations. "A small pistol with a large bore, very effective at short range" [OED].
pocket pistol produced in the early 19th century by Henry Deringer (q.v.), a Philadelphia gunsmith.