'water-excreting' originates from a modern English compound combining 'water' and the present participle 'excreting' (from the verb 'excrete'), where 'water' comes from Old English 'wæter' meaning 'water' and 'excrete' traces to Latin 'excret- / excribere' (via Medieval Latin) meaning 'to separate out or discharge'.

'water' changed from Old English 'wæter' into Middle English and then modern English as 'water'; 'excrete' is from Latin roots (seen in Medieval/Modern Latin forms) and entered English as 'excrete' with the participle 'excreting'; the compound 'water-excreting' is a modern productive formation combining those elements.

Initially the components simply referred separately to 'water' and the act 'to excrete'; over time the compound has come to be used adjectivally to describe things that actively emit or discharge water (i.e., 'releasing water').