'moistness' originates from Old French, specifically the word 'moiste', which in turn is ultimately related to Latin 'mustus' (related to 'must' and wetness).

'moistness' changed from Middle English 'moistnesse' (from Old French 'moiste') and eventually became the modern English word 'moistness'.

Initially associated with words for 'must' or juiciness/wetness in Latin and Old French, but over time it evolved into the current meaning of 'the state of being slightly wet'.