Difference between Underhand and Underhanded

UNDERHAND vs UNDERHANDED: ADJECTIVE

Marked by deception
Executed with the hand brought forward and up from below the level of the shoulder; underarm.
Dishonest and sneaky; underhanded.
Secret; clandestine; hence, mean; unfair; fraudulent.
Done, as pitching, with the hand lower than the shoulder, or, as bowling, with the hand lower than the elbow.
Dishonest and sneaky; done in a secret or sly manner
Thrown etc with the hand brought forward and up from below
With hand brought forward and up from below shoulder level

Done by moving the hand (and arm) from below.
Insufficiently provided with hands or workers; short-handed; sparsely populated; obsolete in this sense, short-handed or understaffed being the preferrred term.
Sly, dishonest, corrupt, cheating.
Underhand.
Acting or done in a deceptive, secret, or sly manner; dishonest and sneaky. : secret.
Marked by deception
With hand brought forward and up from below shoulder level
Underhand; clandestine.

In a sly and secret way.
With an underhand movement.
In an underhand manner; thrown with the hand no higher than the shoulder and the palm turned upward during part of the pitch; -- said of pitching or bowling a ball.
Slyly and secretly
With the hand swung below shoulder level'
In a sly, sneaky or secret manner

By secret means; in a clandestine manner, and often with an evil design.
By fraud; by fraudulent means.
Secret; clandestine: usually implying meanness or fraud, or both.
Sly; contriving; deceitful.
Performed or done with the knuckles of the hand turned under, the palm upward, and the thumb turned from the body: as, underhand bowling in cricket.

Insidious, Dubious, Fraudulent, Unfair, Sly, Shady, Deceptive, Deceitful, Devious, Crooked, Underarm, Corrupt, Underhandedly, Sneaky, Underhanded

Blatant, Deceptive, Disingenuous, Unethical, Duplicitous, Dishonest, Devious, Deceitful, Overlapping, Sly, Underarm, Crooked, Corrupt, Sneaky, Underhand

Insidious, Dubious, Fraudulent, Unfair, Sly, Shady, Deceptive, Deceitful, Devious, Crooked, Underarm, Corrupt, Underhandedly, Sneaky, Underhanded

Blatant, Deceptive, Disingenuous, Unethical, Duplicitous, Dishonest, Devious, Deceitful, Overlapping, Sly, Underarm, Crooked, Corrupt, Sneaky, Underhand

They have no concept of customer service and make underhanded deals.
AOL's customer service practices were "outlandish and underhanded," Blumenthal added.
Power plays, difficulty relinquishing control, and underhanded tactics will backfire.
Or the loathsome, underhanded monster we came to love?
There is nothing hidden or underhanded in our views.
What a shady, underhanded, and downright spiteful trick!
This is actually much more sinister and underhanded.
Not with these underhanded tricks during divorce proceedings.
That is an underhanded way to do business.
Suffragists repeatedly accused antis of using underhanded tactics.