Tautness is from taut + -ness. Taut is from Middle English tought, taught ('tight, distended'), of uncertain origin. Important recorded forms include tought (Middle English). Morphologically, the native -ness suffix turns an adjective or quality word into an abstract noun naming the state or condition. The wording preserves that uncertainty: the older history is best read as a cautious account rather than a fully settled chain. The modern sense is largely compositional, with the historical elements narrowing into a stable English lexical item. In current use it covers the state of being stretched tight or tense. The wider family includes related words such as taut and tauten.