underhand

1. Marked by or done in a deceptive, secret, or sly manner; dishonest and sneaky. See synonyms at secret.
2. Sports. Executed with the hand brought forward and up from below the level of the shoulder; underarm: an underhand pitch.

1. With an underhand movement: Throw the ball underhand.
2. In a sly and secret way.

Sly, deceitful; perhaps derived from card cheats who deal from the bottom of the deck.

adjective

Marked by treachery or deceit: devious, disingenuous, duplicitous, guileful, indirect, lubricious, shifty, sneaky, underhanded. See honest/dishonest.

Definition: deceitful
Antonyms: aboveboard, forthright, frank, honest

The adjective has 2 meanings:

Meaning #1: (sports) with hand brought forward and up from below shoulder level
  Synonyms: underhanded, underarm
  Antonym: overhand (meaning #1)

Meaning #2: marked by deception
  Synonyms: sneaky, underhanded

The adverb underhand has 2 meanings:

Meaning #1: slyly and secretly
  Synonym: underhandedly

Meaning #2: with the hand swung below shoulder level'
  Synonym: underarm