What does Boxing Day have to do with boxing?
1590s, "immodest, wanton, saucy," from Middle French petulant (mid-14c.), from Latin petulantem (nominative petulans) "wanton, froward, saucy, insolent," present participle of petere "to attack, assail; strive after; ask for, beg, beseech" (see petition (n.)). Meaning "peevish, irritable" first recorded 1775, probably by influence of pet (n.2). Related: Petulantly.