A right cross was a type of right-handed boxing maneuver.
In 2267, Dr. Leonard McCoy struck Eleen after she hit him twice first, which finally convinced her to allow him to treat her. Later, when asked by James T. Kirk if he had administe'red a happy pill, he said, "No, a right cross." Kirk quipped that he'd never seen that approach in a medical book, to which McCoy responded that it was now in his. (TOS: "Friday's Child")
In 2369, Benjamin Sisko reminisced with Jadzia Dax about a time when former Dax symbiont host knocked him unconscious with a right cross before Sisko could get in a fight with an Argosian lieutenant who threw a drink in his face. (DS9: "Dax")