What does Boxing Day have to do with boxing?
c.1600, "to burn with a hissing sound," perhaps a frequentative form of Middle English sissen "hiss, buzz" (c.1300), of imitative origin. The figurative sense is attested from 1859. Related: Sizzled; sizzling. The noun is first recorded 1823.
A homosexual who is particularly and nearly exclusively concerned with the length of penises (1972+ Homosexuals)