Dishonest, sneaky, or deceptive; done secretly or unfairly without being obvious.
From under + hand, originally a sporting term describing a throw where the hand stays below shoulder height, which was considered less powerful and thus 'sneaky' or tricky.
An underhand throw in tennis or bowling is actually legal but looks weaker—the term became associated with dishonesty because people used it to disguise their true intent, making it hard for opponents to react.
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