Underhandedness is from underhanded (under- + hand + -ed) + -ness. The figurative sense of 'secret, sly' derives from the literal notion of a movement made with the hand kept low and concealed. Important recorded forms include -ness (Old English), under (Old English), and hand (Old English). Morphologically, the under- element contributes the sense of beneath, lower rank, concealment, or insufficiency, depending on the base it modifies. The semantic development is already visible in the history: an older concrete or specialized use was extended into the modern meaning rather than replaced by a separate origin. In current use it covers the quality of being secretive, sly, or deceitful. The wider family includes related words such as underhanded, underhand, and hand.