1. humiliates him or herself, or
2. fails to complete a task in a spectacular, yet comical, manner.
Derived from an ex-Rangers football (soccer) player, Filip Sebo, who was an utter failure with 'ra Gers in his only season with the team.
2. You are at the beach. You and a group of friends are tossing and punching a beach ball up in the air, when one of them swings and completely misses the ball, resulting in the ball hitting the ground. This is the perfect spot to yell out "Seeebooo! Seeebooo!" at the said friend.
(Particularly effective when the person at the end of "Sebo" is in no position to say anything against it.)