A sore loser is someone who loses in a fair competition but whines about it on a constant basis, blaming everyone around them for their loss except themselves. Fun to taunt, but no fun to play with.
Someone who can't simply be honorable, by accepting defeat and/or trying again. On the contrary, said individual or group engages in childish pissing and moaning; bitching about how it's not fair and the other side cheated, etc.
Definitions from Urban Dictionary.