Heavyweight Shannon “The Cannon” Briggs can add one loss to his nearly-impeccable fighting record — because he just suffered a trouncing in court … for nearly half a million bucks. Shannon’s ex-publicist Kali Bowyer sued the boxer, claiming she helped him earn $2.8 million back in 2009 — but was never paid commissions. Bowyer claimed Shannon owed her $420,000 for her services — and according to a South Carolina court, the boxer blew off the lawsuit. The court issued a default judgment against the boxer this week for the full amount … ordering him to pay up stat. Calls to Shannon weren’t returned.