Shannon 'The Cannon' Briggs -- K.O.'d In $420,000 Legal Bout

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.