Definition from Wiktionary, the free dictionary
Presumably because of tendency to bleed when battered, tomato juice leaking from a can being a metaphor for bleeding.
- (slang, boxing, derogatory) An inferior fighter in the sport of boxing. The insinuation is that an up-and-coming fighter takes on a "tomato can" to burnish his reputation and record.
- (slang, boxing) A boxer who takes a dive, or loses a fight on purpose.