'moisture-retaining' is a modern English compound from 'moisture' + 'retaining'. 'retain' ultimately originates from Latin 'retinere', where 're-' meant 'back' and 'tenere' meant 'to hold'.

'moisture' developed in English from Middle English 'moysture' (moist + -ure). 'retain' entered English via Old French (retenir) and Middle English forms before becoming modern English 'retain'; the compound 'moisture-retaining' is formed productively in modern English by combining the noun and a present-participial verb form.

The compound originally and still means 'holding moisture'; the basic sense has remained stable, describing materials or substances that keep water from escaping quickly.