What does Boxing Day have to do with boxing?
1905, from hiss + -y (2). Hissy fit is attested by 1983.
A noisy fit of anger; catfit, conniption fit, duck-fit: the vile, know-nothing hissy fit loose in our land/ and write that memo before the boss has a hissy fit that registers on the Richter scale
[1990s+; hissy in the same sense is found by the 1940s as a Southwestern usage]