What does Boxing Day have to do with boxing?
late 15c., "caustic" (of words, speech), from Middle French mordant, literally "biting," present participle of mordre "to bite," from Latin mordere "to bite, bite into; nip, sting;" figuratively "to pain, cause hurt," perhaps from PIE root mer- (2) "to rub away, harm" (see smart (v.)). Related: Mordantly. The noun sense in dyeing is first recorded 1791; the adjective in this sense is from 1902. Related: Mordancy; mordantly.
mordant mor·dant (môr'dnt)
Serving to fix colors in dyeing. n.
A reagent, such as tannic acid, that fixes dyes to cells, tissues, or other materials. v. mor·dant·ed, mor·dant·ing, mor·dants
To treat with a mordant.