Definitions of underhand:

• adjective:   with hand brought forward and up from below shoulder level

  Example: "An underhand pitch"

• adjective:   marked by deception

  Example: "Achieved success in business only by underhand methods"

• adverb:   with the hand swung below shoulder level'
• adverb:   slyly and secretly

  Example: "Mean revenge, committed underhand- John Donne"