The quality or state of being gainful; the capacity to produce profit, advantage, or income.
From 'gainful' plus the abstract noun suffix '-ness' (from Old English '-nes'), which converts adjectives into nouns representing abstract qualities or states.
The noun 'gainfulness' is rarely used because people find it too clunky compared to just saying 'profitability'—it's a reminder that not every grammatically correct word survives in actual usage.
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