"Scrappy underdog" refers to a person or team that is considered an unlikely or disadvantaged contender, but puts up a determined fight and often manages to overcome obstacles or achieve success. Full definition
It's hard not to get swept up by the film's progressive zeal, but Disney doesn't allow for much grey area: Beharie and her lawyers are scrappy underdogs fighting for what is good and right, and the D.A. and his cronies are sniveling villains. (film.avclub.com)
Bliss's story of «scrappy underdog asshole» who was only able to get in offense while cheating made the fight between them not so one sided, and the moments where Nia was on offense had Bliss bumping like a boss. (sbnation.com)
12 Years a Slave is the actual literal scrappy underdog that could. (awardsdaily.com)