WordWeb Online

1. With hand brought forward and up from below shoulder level
   "an underhand pitch"; "an underhand stroke"
   - underhanded, underarm
    
2. Marked by deception
   "achieved success in business only by underhand methods"
   - sneaky, underhanded

1. Slyly and secretly
   "Mean revenge, committed underhand"
   - underhandedly
    
2. With the hand swung below shoulder level
   - underarm

See also: bent [Brit], corrupt, crooked

Antonym: overhand

Encyclopedia: Underhand