'magnesium-deficient' originates from combining the element name 'magnesium' (from New Latin 'magnesium', ultimately from Greek 'Magnēsia', a place name) and the adjective 'deficient' (from Latin 'deficiens', present participle of 'deficere').

'magnesium' evolved from Greek 'Magnēsia' → New Latin 'magnesium' and entered modern English as the element name; 'deficient' comes from Latin 'deficere' (to be lacking) → Medieval Latin/Old French forms → Middle English 'deficient', leading to the modern adjective. The compound form is a modern English combination of these two elements.

Individually, 'magnesium' referred to the region and later the chemical element, while 'deficient' meant 'lacking'; together the compound has come to mean 'lacking in magnesium' in contexts such as soil, diet, or organisms.