WordWeb Online

1. (sport) with hand brought forward and up from below shoulder level
   "an underhand pitch";
   - 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
   "throwing a ball underhand";
   - underarm

See also: bent [Brit, informal], corrupt, crooked [informal]

Antonym: overarm

Encyclopedia: Underhand