Definition from Wiktionary, the free dictionary
- Done by moving the hand (and arm) from below.
- Sly, dishonest, corrupt, cheating.
- His underhanded trick backfired and he was disqualified.
- An underhanded compliment is actually criticism.
Usage notes 
- Nouns to which "underhanded" is often applied: tactic, way, dealing, method, trick, manner, thing, deal, scheme, practice.