Someone who cannot accept losing and will bitch and moan while making every excuse while sporting out conspiracy theory even if all the evidence against them is overwhelming
Donald Trump: I only lost because they cheated!
Every sane person: Do you have any evidence to support your claim?
Trump: Just look
Everyone: We did and all of your claims have been proven false
Trump: Fake news!
Everyone: FFS. Such a sore loser!
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.