We really need context. Underdog = the side in a game that is expected to lose Example: China plays Argentina at soccer, China is the underdog. An ultimate underdog is a someone who is NEVER expected to win. Again, the Indian soccer team is an ultimate underdog.

"Underdog" means a person, team, country, etc. that is thought to be in a weaker position than others and therefore not likely to be successful. "Ultimate" in this context means "the weakest"