'fluid-filled' originates from English, specifically the combination of the noun 'fluid' and the past participle 'filled', where 'fluid' ultimately comes from Latin 'fluidus' meaning 'flowing' and 'fill' comes from Old English 'fyllan' meaning 'to fill'.

'fluid' passed from Latin 'fluidus' into French as 'fluide' and then into Middle/Modern English as 'fluid'; 'fill' developed from Old English 'fyllan' to Middle English 'fillen' and 'fill'; the compound adjective 'fluid-filled' is a modern English formation combining these elements to describe something filled with fluid.

Initially the elements referred separately to 'flowing' (fluid) and 'to make full' (fill); over time the compound came to specifically mean 'filled with fluid' in contexts such as medicine and engineering.