'fluid-deficient' is a modern English compound formed from 'fluid' and 'deficient.' 'fluid' ultimately comes from Latin 'fluidus' meaning 'flowing,' and 'deficient' comes from Latin 'deficere' (via Late Latin 'deficiens/deficient-') meaning 'to lack' or 'to fail.'

'fluid' passed into Old French as 'fluide' and then into Middle English as 'fluid.' 'deficient' developed from Latin 'deficere' -> Late Latin 'deficiens/deficient-' and entered English via Old French/Late Latin influence. The hyphenated compound 'fluid-deficient' is a modern English formation combining these two elements to describe a lack of fluids.

The components originally conveyed 'flowing' (fluid) and 'lacking/failing' (deficient); combined in modern usage they specifically denote 'lacking necessary liquids,' most often bodily fluids.