What does Boxing Day have to do with boxing?
early 15c., from Latin constrictus, past participle of constringere "compress" (see constrain). A direct borrowing from Latin of the same word which, via French, became constrain. Related: Constricted; constricting.
constrict con·strict (kən-strĭkt')
v. con·strict·ed, con·strict·ing, con·stricts
To make smaller or narrower especially by binding or squeezing.