Use the adjective underhanded to describe someone who plays dirty tricks, like a cheating poker player or a manipulative classmate.
It would be underhanded to spread gossip about your opponent during a student council election, just as it's underhanded for a business owner to pay herself more while laying off workers. If it's unfair and sneaky, it's underhanded — unless you're pitching a softball, in which case you're simply throwing it from below your shoulder instead of above.