Cheap means using a tactic that is allegedly not 'tournament-caliber', but still winning with it. This may include tactics that resort to low blows, deception, attacking when the opponent is distracted (or causing a distraction), or using a sure-fire attack that is deemed by some to be "too powerful" to be used by advanced players.

Examples of Tactics That Have Been Claimed to be 'Cheap'

SSB
Infinite throw trap

SSBM
The kamikaze glitch

All Games
Kirbycide
Camping Stale Moves/Spamming