What does Boxing Day have to do with boxing?
as an exclamation, "well done!," 1761, from Italian bravo, literally "brave" (see brave (adj.)). Earlier it was used as a noun meaning "desperado, hired killer" (1590s). Superlative form is bravissimo.
It is held by some philologists that as "Bravo!" is an exclamation its form should not change, but remain bravo under all circumstances. Nevertheless "bravo" is usually applied to a male, "brava" to a female artist, and "bravi" to two or more. ["Elson's Music Dictionary," 1905]