What does Boxing Day have to do with boxing?
1945 (n.), slang shortening of schizophrenic. Schiz in same sense is from 1955 as a noun, 1960 as an adjective.
word-forming element meaning "division; split, cleavage," from Latinized form of Greek skhizo-, comb. form of skhizein "to split, cleave, part, separate," from PIE root *skei- "to cut, separate, divide, part, split" (see shed (v.)).
schizo- or schiz-
Split; cleft: schizotrichia.
Cleavage; fission: schizogenesis.
(also schiz) A schizophrenic: Docs find he's a schizo (1945+, variant 1955+)
[fr schizophrenic, schizophrenia]