'water-retaining' is a compound formed from the noun 'water' and the present participle 'retaining' (from the verb 'retain'). 'retain' originates from Latin 'retinere,' where 're-' meant 'back' and 'tenere' meant 'to hold.'

'water' comes from Old English 'wæter' (from Proto-Germanic *watar). 'retain' came into English via Latin 'retinere' and Old French 'retenir' before appearing in Middle English; the compound 'water-retaining' is a modern, transparent compound formed from these elements.

Originally the elements referred literally to 'holding back or keeping water'; over time the compound came to be used more generally to describe materials or soils that 'hold' or 'store' moisture (i.e., 'able to retain water').